Sounds mean, but you may want to invest in a set of hoof Xrays to see how much rotation of his coffin bone he actually has inside the hoof. You say severely foundered, which could be bad enough that tip of coffin bone is actually coming thru his hoof sole. That bad, bone exposure, would mean he really needs to be put down.
Being kind, doing things to improve the plight of rescues is a great thing. But when they come in with serious conditions, you need to be realistic, not stretching out something that is keeping him in pain.
Founder has a lot of ways it can go in being managed or really taking him downhill. Sudden changes of feed, extra feed he is not used to, all can be triggers for another founder episode. This is where the Xrays will tell you EXACTLY how bad he is inside the hoof. He could be just a bit rotated, not nearly as bad as described, or he could be bad as described above.
I would agree that having been foundered, pony should be kept "trim" in body condition. This could mean his ribs are not buried under fat layers. A lighter body is an easier load for him to get around on his bad hooves. I would want to feel his ribs as you would on a very fit athlete, not a porky show horse. No fat neck crest or fat pads above his tail head. He will need to be kept confined so he is not hogging down new grass or allowed free grazing that will trigger another episode of founder. Small amounts of hay fed, no free choice hay bales.
I just want to save you grief later, if he is really bad to start. Don't drag the pain out for him, and the Xrays will tell you. Some animals are totally stoic in dealing with bad pain, because it is the only way they can manage it. Too sore to choose which leg to limp on. It is not a kindness to leave a foundered animal standing on those painful hooves with no sole depth or let him continue moving about with bones exposed to the hard ground. Painkillers are NOT the way to go with this. He would need putting down as the best kindness you can give him.