OK, I'm not an expert; I do have some experience in this though. What I see is a lot of excess sole and trimming needed on the frog. As well, the heels seem high. On the face of it, none of this, in my opinion, would actually make him lame, but he may well have thrush hiding deep within which could be causing trouble. I think in general those feet need to be opened up. It actually looks like the sole has grown in to compensate for something, but maybe it's just still overgrowth that wasn't looked after.
To be expected that he would be sore if his feet were neglected for so long. Hard to say for sure though, since we can't see what his feet were like before. It would be nice if your trimmer was close by and could come every week for two or three weeks to take just a bit at a time. A horse should feel better after a trim, not worse. Subject to just simple tenderness that should go away in a day when in pasture only. Use boots for rough ground if needed.
Even this pic doesn't tell the whole story. Side, front, back and general confirmation give more infro.
And, welcome to the forum!