Welcome to the forum. Sorry to hear your little guy isn't doing well.
I suspect your betta may have some sort of chemical poisoning over long term due to very infrequent water change. Their organs can become slowly affected and damaged. If you said you underfeed your betta, so the obvious answer is that the bloating isn't really caused by food directly.
If it is a fish bowl, the common type is about a gallon? Try to use a gallon clean milk jug and fill it up with water, and pour into the bowl to measure the size. It is always good to know exactly what size tank you have, especially in the event you need to start treatment like now.
Because he has signs of bloating and unable to swim, you need to start him on treatment with Epsom salt. You can get this from Pharmacy, no scent, just plain. Here are the steps:
1) Clean the tank really well with hot water and hand
2) let sit until reach room temperature (I really hope your room temp isn't 60, as that is just too cold for bettas), you need to find a heater for the small bowl, it's more challenging to find, but they are out there.
3) of the 1 gallon water, take a little out and put it in one of those clean water bottle if you have (ie like poland spring disposable), then add 1 teaspoon of Epsom salt into the bottle and shake well.
4) after you check to make sure all salt crystals are dissolved, pour that salt solution into the tank
5) You also need to buy dechlorinator, you can't just use unconditioned tap water. Most people recommend Seacheam Prime conditioner.
6) repeat the above steps every day for the next 10 days or so while watching for other symptoms.
Once you figure out how big the tank is, and need clarification regarding the treatment, let us know.
The number one rule to making sure your betta is healthy and thriving, is to keep the water clean and free of abnormal level of unwanted chemicals.