Sounds like your guy is seeing some changes in topline/condition perhaps primarily because he's not in work, and that's to be expected. I'm presuming he has adequate shelter from the elements so he isn't burning calories trying to keep warm or dry in the winter months. Nineteen isn't super old, and if he has the ability to chew and eat without issues, you wouldn't necessarily need to go the "senior" route yet with his nutrition.
Tractor Supply sells their Dumor brand alfalfa/bermuda combo hay cubes in 50# bags. This provides a bit of alf plus the necessary forage, whereas alf pellets alone don't provide the same forage value. Forage first.
My 26 y.o. retiree gets free choice coastal hay, pasture turnout (varying degrees of palatable grasses), and about 5#/day total of Senior Glo (ADM), given as two meals. I fed Triple Crown Lite for years until I couldn't get it around here anymore. At night check he gets about 2# of the bermuda/alfalfa hay cubes (
TSC), which have been thoroughly soaked. Provides some more water and the forage he needs. He thinks he's getting another meal
I personally wouldn't go the oats route. Very high in non-structural carbs, which have been implicated in the whole Cushings/insulin-resistance spectrum of problems.
If mixing your own feed, what about providing the necessary vitamins and minerals in adequate amounts? That's the good thing about a 'complete' feed--already taken care of.
If just increasing fat calories, you almost can't go wrong with flax seed meal or flax seed oil. Horse Tech and Smart Pak Equine are good sources. The omega 9's are a safe fat source (canola oil, rice bran oil). Start with small amounts when adding any oils; increase incrementally over several weeks.
As with any change, only take on one thing at a time. If you implement multiple nutrition changes at the same time you'll never get a handle on which new foodstuff/supplement helped your goals. And all changes require lots of time to see results, i.e. months. Which then puts you into springtime again.
