If there's any softer terrain for him to move around on, that may also help keep him more comfortable for now.
Do you think he may have had a reaction to any vaccines, dewormer, or something else? I wonder if the salivating and lameness may be connected.
How he moves in the videos (thank you for uploading them to show us) gives me the impression that it may not be laminitis although with his appearing heavy he could be a candidate for it. There may be the onset of low ringbone? As stevenson suggested it could be soreness or a possible bruise or abscess. Maybe a mixed bag of issues?