I, too, suspect an obstruction in the furnace burner tube, possibly mud dauber nests, maybe spider webs, or even just a pile of dead stink bugs. This is also likely the cause of the furnace coming on for just a minute before shutting off, only to come on again for just a minute a brief time later.
You would be doing him a favor to tell him you think there is a problem. If that burner tube is obstructed, it is also quite likely making carbon monoxide in prodigious amounts, which could kill him (or you). At the very least, it will be wasting a lot of propane by a burn cycle like that.
Last spring, the first time I fired up my TT's water heater, it sounded like a jet engine. It had always had a faint roar, but this was impressively loud. I went outside to investigate, and that's when I saw the big black carbon plume going up the side of the trailer. I shut the water heater off immediately and quickly determined that I had a burner tube full of dead stink bugs. Took all of a minute to clear them out, whence the water heater returned to normal operation. Took some serious scrubbing to clean the soot off the side of the trailer.