Is it ethical to tell my teaching assistant that I like him? I, a twenty-year-old female, have come to admire the graduate teaching assistant for my summer course quite strongly. I'm extremely interested in what he has to say, I always pay attention, never use my phone and use eye contact to show interest. I do like them very much and I think about them with warmth often. I want to know about their work, I want to know what he is interested in. I would like to at least be their friend.
Despite my strong admiration I must emphasize that I have not and do not plan to cross professional boundaries until the end of the course.
I have spoken to them though it was only a short conversation about the work that was assigned to us the following weekend, however he did seem quite warm and open to conversation. Specifically he emphasizes in his syllabus to not hesitate to contact him with any questions we may have at all. I would like to be able to talk to them more without bothering them, and over stepping professional boundaries. I hopefully plan to ask them out once the course has ended, but I would still like to be able to have the chance to talk to them too as friends outside of course hours about the course material. Would this be ethical? Should I just back down? Specifically I wanted to thank him for giving me a very positive, in-depth feed back in the essay assignment that I had turned in that I got a perfect grade on.
I've been going through a really rough time in my personal life. Despite having flunked before, I made a promise to myself to work harder and to never give up on my goals. Truly their receptiveness, warmth and passion has genuinely inspired me to keep going and to study harder. I would very much like to thank them for inspiring me and being a good teacher which has helped me understand the material.

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

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


*edit
Class has ended and we have a date planned! Thanks all for your advice!
*edit #2
We are still dating and our 9 month anniversary is coming up soon! The relationship is going really well and it is the best and most loving relationship we’ve ever been in! We are excited to grow together and the mention of having children someday has come up occasionally.
Sometimes he jokes about being my TA but he is very adamant about making it clear he didn’t like me during the course, but in conclusion we are both doing fantastic!
Thanks for all your help ! (May 7th 2020)
*edit # 3
Not sure if anyone comes across this anymore but our one year anniversary passed a few months ago and we're growing strong! I'm really happy that I went out of my comfort zone and found a lasting relationship. (10/28/2020)
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.