I have a feeling the fish is refusing food because of the lack of clean water. 1g need to be changed every 2-3 days to prevent ammonia build up. I would try increasing water changes.
If the fish is not bloated, it is likely not constipation and an epsom salt bath is only going to do harm.
I have a feeling the lax water change schedule has taken it's toll over the last year and as the fish is aging, he can't handle the large ammonia build up. Try fresh clean water. Do a 100% change every other day and 50% change on the off days. I have a feeling this might turn it around. Clean water also helps a constipated betta pass.