This question is as straightforward as it sounds. Do I have the right to demand that he stay out of my projects due to fear he'll not know what he's doing?
He basically googles things and tries them out. This sounds like what I went through in my beginning bootcamp days. I now have way more expertise obviously.
A healthy software development lifecycle will mitigate the pain of an unsupervised junior developer (which is what it sounds like your boss is) trying things out for the first time.
An incomplete list of crucially important things to have are:
The main theme there is testing. I strongly suggest you review your development process and start doing more of it. When you've got it to a reasonable level, you won't be bothered about your boss contributing to the project - the tests will tell you if anything has broken.
The objection to including more testing in your lifecycle is going to be "we don't have time for that". The reason I'm sure you don't have a sufficient level of testing at the moment is that you're worried about one developer messing things up in ways that are hard to find and fix. Testing and reviewing will drastically decrease this risk, and is really, really important to the stability and reputation of the project. You don't have time not to test.
my projects
A really important lesson to learn in professional software development is that they're not actually "your projects" at all. They belong to the company.
Not a programmer, but I had a boss (I work in Supply Chain, Purchasing) who oversaw, purchasing, logistics, customs and internal transfers (basically our internal supply chain). He wanted to learn how to purchase. So what we did was gave him a "dummy" system (complete with the real reports) and let him fiddle and he then worked out the min/max, PO amounts, etc. and sent us his work to see if he could do it. (He did have a good reason to want to learn the basics after never doing it in his almost 30yrs in logistics). Maybe something like this would work for you
Do I have the right to demand that he stay out of my projects due to fear he'll not know what he's doing?
You could ask your boss that (which would be a bad idea) but at the end your manager is... well, your manager, so they can involve themselves as much as they want in the projects they give you.
You say that you "fear he'll not know what he's doing", but that is speculation; it does not guarantee that your boss will fail. Chances are that your boss, although not technical, may know their way around code better than you assume.
For example, my boss (the CEO, not programmer by profession) sometimes comes to me with some code snippets or in some cases links to SO or some blog. Quite some of the things my boss showed me were beneficial to the project we were on.
Try to see what approaches or solutions your boss comes up with, test and judge them (best case some of them work perfectly), and only after that proceed to propose and show a better alternative.
If your boss or manager wants to do some programming, there is no reason to try to stop them.
If they want to do some programming, and expect it to go into the company's products or production code, and if it is your responsibility that your products or production code are of high quality, then their code must be reviewed, and rejected if it has problems, until these problems are fixed. Just like your own code, or anyone else's code. And with the same quality requirements for acceptance.
Now if you have a boss or manager who thinks a bit of programming can't be that difficult, and his code should be put into production, no questions asked, then you have an idiot as a boss or manager. If it's a manager you might be able to do something about it. If it's the boss, analyse their code, figure out what visible bugs it will cause, and ask QA politely to check for all these bugs and report them, assigned to the boss. You can then hope that this will be educational enough. Otherwise, find a new job before the company goes under.
If you're looking for assurances that you're entitled to tell you boss what to do because it's your domain and it's his problem you're going to be disappointed. He's putting you in an awkward position and there's no easy way out for you.
If you have a mature development team with proper testing, code reviews and automation then you're somewhat protected from what is essentially a junior developer wreaking havoc but that doesn't sound like the case.
Again I don't know you or your boss' titles or the structure of your team/company but if this happened to me I would start looking for a new job. What he's doing forces a conflict of interest on you and is obviously inappropriate. If he's a manager (you called him your boss so I'm assuming he is) then this screams incompetence, and being your manager, he's the most important person to your success and happiness at the company. Not a good situation.
