Is it ethical to tell my teaching assistant that I like him?

Wait until after the course is over and grades are in: don't put your TA in a difficult situation.

After that, you are just two adult humans, assuming you won't have any other courses with this TA.

I'd like to add that there is another reason for waiting until the class have been over for a while. There's a phenomenon of "love for authority." It happens a lot in academia, where the instructor would otherwise not be that interesting, but because of the moderate position of power (and the things that go with it, like being the perceived leader, self-confidence, etc) he is much more attractive. It could be that a month after the course is over, your TA will fade back into his proper level.

Love relationships based on an unequal balance of power are usually unhealthy. You probably want to make sure that you're really attracted to him and not just his position.

Consider the other person's feelings in this as well. They could fall for you based on your attraction to them. If you find out later that your attraction was solely or mostly because of the teacher-student dynamic, it could be pretty hard for them when you break it off.

Would it be ethical to express these sentiments of admiration and gratitude to them during their office hours?

Of course. I would wait until the course, and the grades, are finalized, though. Consider sending a note or expressing your (professional) sentiments during TA evaluations (if such a thing exists where you are); doing it in office hours might be uncomfortable.

Would it be possible to date them after the course has ended since they are essentially a normal graduate student instead of a true professor ?

You would have to check your university's rules. In general, there are no policies prohibiting this sort of relationship, if you will have no future courses together. The TA in question might be concerned about the appearance of impropriety, however.

As an artist, would it be strange to ask them if i could give them a drawing?

Of course you should not give them anything until the course, and the grades, are finalized. As to strangeness, that's maybe an interpersonal issue -- for me, yes, I would certainly find it strange, but that's not to say I wouldn't think it was awesome, especially if I liked the student.

It gets more complicated since you plan to ask him out though -- if you give it, then ask him out, that's a bit uncomfortable; if you ask him out and are turned down, it's a bit awkward to give him the artwork anyway.