Why is my betta not eating fish food?

I have my betta in a non-permanent 1.5 gallon tank (it has a light, java fern, and barely bubbling airstone - for circulation - in there) since June 18th. I have fed him only 1 blood worm so far, and that was two days after I got him. This past weekend I did a 1/2 gallon water change w/ siphoning to clear up anything that might have gone to the bottom (nitrate, nitrites, and ammonia were perfect). Yesterday I noticed that he was pooping alot (and each poop was bigger than a betta pellet!), so constipation was probably his issue at first. Today he was moving around alot more, so I tried a blood worm (to see what would happen) and he went up to it, kind of sniffed it, and then swam away like he was disgusted. His belly looks healthy, so do you think it is more constipation? I tried to feed him part of a peeled boiled pea, but he wouldn't touch that either. Thanks for helping.