Do you remove the food when he doesn't eat it? If not, he might be eating some when you are not looking.
Bettas can easily go 2 weeks without any food so the fact that your little guy is nibbling a bit every now and then when you try to feed him is good. Bettas in cooler temperatures sometimes will not eat or will eat very little. I suggest raising the temperature to around 80F and see if he reacts any different to the food. Also experiment with different foods. Some bettas are incredibly picky about which pellets they like and it can take a little while to learn which one they prefer. You can also offer (like suggested above) some frozen brine shrimp or bloodworms. I also suggest live foods such as white worms, mosquito larvae (when available), or chopped black worms.
It is important to really watch your water parameters to make sure they are stable too. Ammonia spikes could cause him to go off eating as well. Another thought, how does his poop look (if you have seen it)? If it is white or stringy, he might have parasites which can cause them to go off of food too.