Puffer Digestive Trouble (Diet Q's?)

Hello TRT. I've researched high and low for a good answer, but I've drawn a blank at every turn, and I'm hoping someone who's had the same problem will have the solution! The problem seems relatively mild, I have a porcupine puffer, Porky, who's not very old (I'd say ~1 year), still small for his species, only about 4-5in from tip to tail. I've been feeding him a pretty standard diet of .5-1 silverside, 2-3 clams, and 3-7 peas every one to two days for the whole time I've had him (8 months in a few weeks!), and only since arounf January has he developed a problem. It started out as a general lethargy, and a tendency to 'dock' between my tank heater and filter, so that he was stuck to an inflow vent and held in place under the heater tube. He very deliberately lined it up, and had no trouble detaching from his dock afterwards. The tank temperature wasn't the issue, it swings between a steady 78-80, and he looked pretty comfortable, so I'm suspecting he was just too lazy to swim. Lately though, it's escalated to hiding in a little rockwork cave all day, except when guests visit or it's time to be fed. Also, most every day after a meal, seems really buoyant and doesn't leave the cave, almost like a balloon, from gas in his tummy. I've tried the standard 'burping' method for removing air from a puffer a few times it's happened, and all it seems to do is make him more uncomfortable. I'm worried that if I puff him up too often he'll hurt himself, especially since it hasn't helped. He seems to have really rough digestive trouble, and definitely suffers from constipation almost every other meal. My question is, why? Are the peas the problem, or is there something else I should be supplementing his diet with? I'm just really worried for the little dude, when he's like this he seems so miserable, and it takes him hours to pass some of these blockages. I've looked at the waste, and as far as I've been willing to look into it, it seems to be a blend of half-digested fish, clams and pea shells, but nothing super unusual. Should I shell the peas before feeding him, or is that maybe not the issue? Could he have a parasite? I'm kind of at my wits end, I'm considering cutting peas out of his diet permanently, but he'd be heartbroken. It seems to be a real issue for the little guy, and I want to do everything I can to give him a happy home. I just don't want my little guy to hurt himself trying to deal with something that's probably my fault. It's not a life-threatening situation yet it seems, but any advice at all is appreciated.