You are talking serious amounts of money, so I'd carry on putting serious amounts of thought into this!
How old is your son? Any hours logged from age 14 can count towards both his PPL and CPL.
He can use 30 hours flying motor gliders to count toward his CPL. An NPPL SLMG converted to PPL would probably be his easiest route to a PPL with his present experience, so there's little point in changing to SEP until he gets that.
Once he has his NPPL SLMG he could convert easily with a few hours at a reputable flying school flying EASA SEP as he will need (for want of a better phrase) Group A SEP hours, not microlight ones to credit towards his CPL.
It's not really worth him buying a permit aircraft until he gets his PPL as it's a mire of regulation about where, who by, and what he can train in to get an EASA SEP PPL (which is what he will need). Yes he could train in his own aircraft, but with what experience he has, he's close enough to get the PPL easily without the confusion of regulations ect.
Once he has his PPL, an LAA permit aeroplane is a good way to go for hour building. For cheapness (and IMHO reliability and safety) buy something like a Jodel or other well proven design (maybe single seater for really cheap) rather than a more modern kit build. They may be older, but they will be well proven and fly safely. If hangarage is a problem go for something with folding wings (some may scoff, but look up Clutton FRED to get an idea of what I mean).
People will come along and say it's not the "right" kind of flying to lead to a CPL, but I would argue that a wide variety of flying is just what a CPL should have! If he's serious, a cheap single seat PFA type is perfect for hour building. A relatively fast one would probably be best (ok, not a FRED lol) as he will need at least one long cross country (ideally several) under his belt for a CPL.
What his instructor suggests has some merit, but like others have said ... It sounds like some self interest is creeping in there! I certainly would not recommend what he/she is suggesting.
Be careful what you decide!
Good luck to him, SS