Try alfalfa pellets, only a handful, soaked, mixed in the feed. Then increase in little steps. Moisten the senior feed, too, and mix very well, he can't pick out the goodies then. Just about all senior feeds contain alfalfa an beetpulp, so, if started slowly, he shouldn't be turning his nose up, really. Maybe it was just too much.
Ricebran, canola oil, flax, all good sources of calories. And, a good senior feed should already have at least 5% fat and he easily digestable. Which one are you feeding, and how much?