Hey guys. :c 

I'm really concerned about my rabbit.

Just the other day I noticed that he has been a bit down, and looks really groggy/sleepy.

And he would kind of just lay there the whole entire day, not moving much, and he started to sneeze. D:

I also checked to really make sure if he was okay by giving him his favorite snack, and he usually jumps up and runs around and goes crazy about it, but this time he just sniffed it and sat back down. :c

So I'm really worried.

Should I check him into the vet? 

Lack of apetite and lethargy are definitely things to be worried about in a bunny. Has he been eating anything at all, or pooping? I think a vet visit would be a good idea.

Today he is acting more normal and he is back to eating his snack like crazy.

:c

So I just wanted to make sure if I should still be worrying about it.

Just try to keep an eye on his apetite and pooping (it helps to empty the litter box so you can see how much he's pooping). If his activity level is back too, then I'd say he's probably feeling better.

I agree with Peppy. Keep an eye on him but if he seems to do it again, I'd take him to the vet rather quickly. Especially if he stops pooping or peeing or if you're noticing he's not eating anything. ESPECIALLY his favorite treats. He may have just had some gas or small bout of tummy not feeling well which he cleared up on his own... if he does it continually though he may be having trouble with food he's getting, not enough hay or water, or some other tummy ailment.