This could be due to so many things, it's hard to tell, but you could explore these with him -- using gentle questions and talking with his teacher (without your son around):
1. It could just be basic, normal five-year-old resistance to what mom and dad want him to do. Yes, he may be great at it, but that doesn't mean he wants to do it every week at the same time. It may be that he does like it but also liked his "down time" over the break and is having trouble getting back into things (I've heard lots of friends say their kids just are so slow wanting to return to anything after the holidays!). If this is the case, you have the dilemma of deciding whether to let him stop or to tell him he has made a commitment to these classes and has to keep going until the end of the session or whenever. You did note he had a good time when there, so you may find he goes more willingly once he sees he will indeed be taken there each week, tantrum or not, and expected to participate.
One thought: You're clearly and rightly proud of his achievements in tae kwon do, but has he possibly been doing it more because you and his dad are so proud of it? It could be he's ready to explore other activities and mix things up a bit but has been reluctant, up to this point, to tell you. I'm just putting the idea out there--it may be he really has a love for it, but it's worth considering if he needs a break (perhaps after fulfilling his class commitment so he learns to stick with it until he's done).
Regarding trying new activities: My friend's 14-year-old began tae kwon do around your son's age and is a black belt in his level and will continue to do tae kwon do and earn more belts--he's very good. But he also takes breaks from tae kwon do and also plays baseball and the trumpet. So letting your son explore other hobbies now won't necessarily mean he will give up tae kwon do.
2. The problem could be something that happened during a class that he never mentioned to you -- he got shoved, maybe accidentally, but thought he was being pushed around, or an older kid made fun of him (or he thought an older kid made fun of him--kids this age can be super-sensitive to what they perceive as slights). Or a teacher spoke sharply to him to correct a form and it seemed like huge deal to your son. Or it could be more serious, like he was genuinely bullied in the dressing room or whatever. I kind of doubt the latter -- he would have probably shown tension or upset sooner. But you never know, and some kids hide things very well even from themselves, so you may have to work on finding out.
Whatever you do, I'd involve the teacher and ask about how your son really seems to be doing in class, if he pals around with the older kids or seems isolated from them, if the teacher sees sudden reluctance like this all the time from kids his age (I'd bet he sees it at least sometimes and even among kids with a gift for the sport), etc. etc. A good "master" of any sport should be aware of each child's needs and should be able to help you. And most of all, this may pass very soon and your son will be back in class and eager to go. I hope so!