If he is just an Israeli citizen, and either, doesn't voluntarily support the atrocities Israel is propagating in Palestine, or if he does, but the support is out of his ability (such as being forced to pay taxes unless he risks imprisonment), then it is fine to work with this man as Allah (The Mighty and Majestic) said in the verse of Surah Mumtahana
("Allah does not forbid you to deal justly and kindly with those who fought not against you on account of religion and did not drive you out of your homes. Verily, Allah loves those who deal with equity.") (60:8)
And it is said in the Tafsir of this verse in Tafsir As-Sa'di that the meaning of the verse is that it is permissible to deal kindly and justly with the polytheists so long as they aren't trying to actively fight you or harm you because of your religion.
However, if your client actively supports what is happening in Gaza, actively giving money to the Israeli government to aid in what they are doing, then you shouldn't work with him, as in the next verse of Surah Mumtahana, Allah clarifies it is the polytheists who kill and drive Muslims out of their homes because of their enmity towards Allah's religion who we are forbidden from dealing with them kindly and justly and actively making bonds with them.
I ask Allah to grant you guidance in this affair, and any good from my answer is from Allah, and any deficiencies is from myself and Iblis, and I sincerely repent to Allah for any mistakes in my speech.