Then I really wouldn't worry about it. Now if he were 15-20 it would be a different story though.
I really think sometimes people way over complicate feed programs and over supplement - which is fine, to each his own, but I think awful expensive! Even a cheap supplement fed throughout the horses life adds up fast - and if it's not necessary it's a waste.
Even at $5 a month, that's $60 a year. Times 10 years is $600! What would be better? $60 a year in unecessary supplements, or $60 a year into a special savings account for equine emergencies, shows, tack, or whatever else? However if the horse needs the supplement by all means they are worth it - for example moms colt with chronically poopy feet looked wonderful after a year of horseshoers secret.
Minis aren't just plain aren't as stressed on their joints as biggies. Less weight on them, no riders, less speed work, etc. a heavily driven horse might be, but they still don't have the stress of a heavy rider on their back - weight is distributed far better in better places for the horse by a cart and harness.