If he’s fat he doesn’t need the extra calories from alfalfa for sure, clean grass hay is certainly enough for my minis (they’re not on pasture for 8 months of the year, just dry lot), if anything extra a basic vitamin and mineral supplement in a carrier of a few grass pellets (mine get 1 big horse supplement scoop worth which is about 2oz!). Use this time when pasture is lacking and the weather is miserable to get some of those extra lbs off so when pasture comes in spring he’s not getting even bigger ?