Do I need to introduce a feed for my New Forest X with EMS?

[Showmum]

He's 18 and we've had him only 3 months (first pony). Currently he's on just grass and soaked hay. His previous owner said she never gave him extra feed over winter as he puts weight on. He is prone to bucking in canter and trot and we think he has issues with his back and back legs. He was out of work for 2 years before we got him and given his age so we are gently increasing the exercise with him. He's absolutly fine in walk and has great manners on the ground and we are getting his back looked at. It was suggested that he might benefit from a supplement for his legs but as he doesn't have any feed I was thinking about introducing a token feed for the supplement but what would be the best way?

Also, as a first time owner so bear with me, if he remains on grass (not much left in field now) and soaked hay, is he still getting all the vitamins, minerals and goodness he needs to see him over winter?

 

[Exploding Chestnuts]

Not sure about his condition, but I would try either Safe and Sound [fed quite damp] or Fast Fibre , soaked to a porridge, as a carrier.
Soaking hay is bound to leach out minerals, and you may need to add a bit of salt. Depends on the fodder, but generally adding a small amount of minerals might be a good idea now.
If you weigh-tape weekly you will have a better idea if weight is going up or down, you dont want to wait till the loss/gain is evident by eye., and adjustments have to be gradual.