Alfalfa hay does generally have more Vit E than grass hay - about 40 IU/kg vs 10 or less for grass hay, according to a study that KER cites. That’s just not a lot. You’d have to have the hay analyzed to find out your specifics, but since that’s not feasible, you shouldn’t assume it has more.
If the mare is eating 20lb (9kg) of alfalfa, that’s 360IU. A mare nursing a 2 month old foal needs 1000IU if she’s 1100lb. 1200 if she’s 1200lb.
At 6 months, the foal might be eating what, 10lb of hay? If that? That’s 180IU He will need 432 IU if he’s destined to be 1100lb, 518 if 1200.
That’s why you need to supplement, based on whatever additional is in the concentrated portion of the diet. It’s pretty hard to OD on Vit E. For the mare, the best thing would be to get her blood tested for Vit E and selenium, particularly since you’re in an Se-deficient area. You can’t assume she’s deficient just because of that though - I’m in a pretty deficient area but mine test normal. If both E and Se are low, get the E levels up first, as that’s necessary to utilize dietary Se. Then if Se is still low, add that in too.
FYI - Foal-Lac is a milk replacer, not a milk-based pellet. It’s for foals who should be nursing still, but don’t have a dam/nursemare. It’s not going to hurt him, but by the time he’s 4-5 months, he’s just not going to need that - he’d be better served with a milk-based pellet for another month or 2. Hopefully his pull on mom will start to let up a bit and you can hold off weaning until 6 months at least - better for the foal all around 
