If you have to give him some that way, it's OK. Be better if you could put it in the food and stir it in though. I wouldn't give too much like that or he'll get the runs.
Is it ok to give my cat olive oil by syringe for constipation, he won't eat it on his food?
Yes giving olive oil to your cat by syringe is fine, just don't give him more than half a tablespoon.
If your cat dislikes olive oil you can use fish oil instead which he might find more pleasant. Try that on his food first as it will be less stressful for him, but if all else fails, the syringe method will be fine.
You can also try canned pumpkin (without sugar or spices so canned pumpkin intended for pies should be avoided) or warm milk, both of which can bring on a bowel movement.
It may be that your cat is dehydrated, in which case, drinking water will help him. That said, I know how hard it is to get a cat to drink!
Olive or fish oil is good for constipation because when a cat is constipated the bowel absorbs the moisture in the feces and this makes it even harder for him to pass it. The oil will lubricate and moisten the feces making it easier for him to have a normal bowel movement.