I had a pony that was prone to laminitis and dropped weight when he got older. Have you checked his teeth? Keep him on low sugar, low protein feed. Alfalfa is generally too rich for laminitis prone horses. We fed teff and timothy pellets with senior feed(with no molasses) and he gained the weight back without incident. Also, think about putting him on a daily laminitis preventative medicine. I mostly gave it to the pony when he got into rich foods, had extra work, or seemed a little off to me.
I didn't read everything, so sorry of there are repeated ideas!