Hmm it does sound like gas or a GI issue. I also recommend taking him to a bunny savvy vet where they can examine him and let you know on his general health, especially since he is having issues now.
Where did you adopt him from? If it’s from another owner or a shelter then you could try asking them what Kuma’s previous diet was like and try to adjust accordingly. If you just found him randomly, the key is to introduce everything slowly. Pellets should be plain pellets without extra stuff like corn etc. If the pellets are colourful then it’s most likely harmful. Oxbow is good and I’ve also heard good things about Sherwood.
Hay should be grass hay like timothy, orchard, meadow, oaten etc. Veggies should also be introduced slowly, one type at a time and in small portions each time so you know which vegetable was the culprit if he has an upset tummy. Some bunnies are not used to vegetables and the sudden surge in this could upset their gut system. You can try stopping the vegetables for now and just put him on a pure hay and water diet to reset his system and reevaluate his diet.