Agreed with all the other posters.
Also, as an aside, even though he's older, he does not necessarily "need" to be on a senior feed. My 27 year old girl is on hay pellets -mostly to stick her supplements to- and a powdered vit+mineral supplement (plus unlimited grass hay, plus a little alfalfa during the winter) and, during the winter when hay pellets+hay alone is not cutting it, I add beet pulp and/or up her alfalfa portion. Beet pulp can really pack on the pounds! :)
As far as making sure his diet is balanced, I'd look into getting him a mineral supplement or a ration balancer. I used to have my girl on a ration balancer but she's such an easy keeper in the summer that the extra sugar was not good. But for your guy, something like Enrich 32 could be great. You generally only feed about a pound a day (making it cheaper than most % complete feeds) and you can add beet bulp or rice bran or hay pellets or a combo for extra "oomph".
Just some ideas. :)
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.