A mix for a native foal wouldn't be a great idea - in order to meet nutritional requirements you'd have to feed a lot. That would mean loads of extra calories for a foal that really does not need them. Why don't you get a broad spectrum vitamins and mineral supplement and add it to a handful of speedi beet or something similar? Then with ad lib hay and grass you'd have a complete diet. I like the look of the NAF Mare and Foal one. Although we have horses, so maybe for a native a mineral block would do just fine.
I'm sure someone who breeds welshies will be on, they'll have a better idea, in the meantime just save your money and don't go buying loads of hard feed! Good luck with your little foal
I'd be very wary of feeding a stud mix of any type to a native pony and think you might do better with either s tud balancer or a lo-cal balancer along with grazing and hay. You want something that will give them steady growth rates although most youngsters do have growth spurts at times. D & H Suregrow will give him everything he needs without the calories of a mix and I feed it alone to my foals at a rate of 100 gm to 100 kg of weight per day so basically a cup of it morning and night.