Boas are supposed to be thin/square, never round or plump like a ball python.
If you can keep the ambient in the low/mid 80's then putting a heavy plastic or stone hide over the UTH may bump the air temp under that hide enough for him to digest.
At some point he likely will outgrow that 55-gal tank, and you'll want to have hiim set up in something designed to maintain his heat and humidity before winter hits. Personally I would go ahead and order his "adult size" enclosure now, something like a 4'x30" PVC Boamaster, NWConstrictors, AP, or similar PVC enclosure w/ RHP for him now, as their lead times are around three months.