I rescued Mudpie at about 200 pounds underweight, and two years later, for the first time ever... he is FAT!
While you're aware of health probs from obesity, as with ourselves, horses can afford to get fat without health probs, but it's retaining weight(don't have to be obese) long term without regular 'hard seasons' to use up the fat stores that tend to cause metabolic & other issues. So it's great that you're aware & good to start feeding him differently because of it, but don't stress!
He's being switched off of alfalfa to a grass/alfalfa mix.
Yes, alfalfa is quite high in energy, protein & calcium, among other stuff, so best not to feed too much of that - about 1/4 of the daily forage ration tends to be considered reasonable - & I'd consider cutting it all together if he's already too fat. BUT grass hay can also be quite high sugar/starch, depending on type, how it's grown, etc - eg most hay is made from 'improved' cattle fattening rye grass, etc. So I would be looking for low NSC grass hay if poss.
Mudpie still needs to be receiving 2% of his body weight in forage (that's approx. 20 lbs). Will grass hay make it okay for me to be feeding him that weight of forage?
It would be much better than receiving it in alfalfa. You can reduce it to 1.5% bwt daily minimum if needed. The most important thing is to ensure the horse has access to hay free choice or little & often, so he doesn't have any substantial periods of hunger. You only mention feeding 2 meals/biscuits of lucerne daily at the moment & this is not adequate if he isn't getting other hay/grazing.
If you're nervous about leaving him with a hay net, there are different kinds of hay bags, or you could buy/make a 'slow feeder' from weldmesh or such, to slow his intake & prevent him gorging.
Re his current diet, we've discussed alfalfa/lucerne, but I'm interested why he gets 2 different omega supps? As the Equus one is high fat I'd ditch that one, for now at least. I'd also divide the other supps up between the meals & feed more frequently if at all possible.
When changing diet, also be aware that nutritional supplements that were appropriate may also need to be changed/adjusted to balance the new regime.