I can't believe I didn't see this thread...
OP( means original poster, by the way
), you said you can get St. Hippolyt. EXCELLENT feed. Reformmuesli is GOOD. They also have THE best hay pellets I have ever seen in 45 years with horses, PRE ALPIN. Get that, if you can, and, since you go there twice a day, give him 2.5kg each time, soaked, so he can eat it easily, and some Reform Muesli with it, about 1kg each feeding.
Other possibility, since the barn provides oats, get Struktur Energetikum instead of the Reform Muesli, and give 1kg a day together with the oats. That will give him all the vitamins and minerals he needs.
I suspect he also needs worming, don't know if you can get dewormers in a pharmacy or have to get it from a vet with prescription( yes folks, EU requires a prescription for dewormers), but ask for one containing Ivermectin and Praziquantel. You should have EQUIMAX, it's available EU-wide.
Have the vet take a good look at his teeth again, he's young and could still have problems with changing from baby- to adult teeth.
Once you have the Pre Alpine and the St. Hippolyt feed, you can cut down and bring him one carrot and one apple a day....he will love you just the same