To accept an answer (especially an up-voted one) does not have severe practical consequences (especially as regards getting additional answers). Mainly, it just changes how things are displayed. One can still add new answers. It also possible to change or undo the accept any time. In that sense it does not change all that much.
However, it sends the signal that the question is answered to OP's satisfaction already. So, they might loose some visibility. Yet, on our site that is rather low volume, this should not be that much of an issue.
As mentioned in a comment, it is possible that OP might have a bit misunderstood what "accepting an answer" means (I have seen cases where askers thought they have to accept an answer as a means to check against spam) or just thought that it was good form to accept an answer quickly.
To answer your questions point by point:
Was it a mistake for him to accept an answer if he wants a variety of answers?
I would not go as far as saying it was a mistake. But it is slightly unusual as it sends mixed signals.
Is there anything he can do to encourage more answers (it's a few days now)?
The main mechanism to get more attention is to place a bounty. However, this is mainly useful on high-volume sites where it can be difficult to get visibility for a question. It might make sense here too, but rather once a question is so old it is not at the top of the page anymore. At the moment this questions still has a top spot in the "active" and "new" list, so there should not be any lack of visibility.
He is thinking of posting an answer himself - will that make it active again?
Posting an answer, as well as editing the question, are a modification of the thread that will place the question-post at the top of the "active" list anew. In this sense "yes," but it will not undo the accept so in another sense "no." But accepting an answer does not close the thread in any way, so it is not really inactive now either.
Will accepting an answer prevent him from getting more sources, since the question has been answered?
No. It is still possible to answer questions with accepted answer.
A thing you could do is to signal to OP that there is no rush in accepting an answer. You could leave a comment along the following lines:
"Thank you for accepting the answer. However, if you are still interested in receiving additional answers it might be better not to accept an answer at this point in time; accepting an answer might suggest to others the question is already completely answer. You can un-accept the answer by clicking the check-mark again; it is possible to re-accept it later."
But as said I think it is not a big deal either way.