I don't know how much to give your little guy, but 50-100 IU's per day is what I give small cats.
We have the oil filled caplets, and I stick the end with a safety pin, then drip the oil on the cat in question's paw. They can't leave it there, and lick it off. If it's double what you want, you can give 1/2, and then finish it the next day. I like the caplets because it's less messy than a bottle of liquid, and is easier, for me, to give the correct dose.
The first time that I gave this, was 28 years ago.
I've not had any problem with it, but can never guarantee an individual cat's reaction to any given supplement, of course.
As long as they don't get high doses, it should be fine, but you can always ask your vet what he thinks.
Please keep us posted on how he's doing.
Do you have a birthday for him?
Love All Life, Thank You For Posting! :)