if he is able to get a Residency Certificate and the rest of the required paperwork, is there any reason why the DLT wouldn't let him sit the tests for a motorbike licence? I thought that, by having the documents (with RC being the key one), it should be purely procedural from that point onwards?
he has his IDP - it's valid for 2 years. The problem is his licence doesn't have motorbike entitlement beyond the AM category, which isn't recognised in Thailand. He can drive a car over there on his IDP for 90 days at a time, which as you say is reset after a border run or flight in/out.
his licence has the AM category entitlement which allows him to legally ride motorbikes up to 125cc on the road after completing a Compulsory Basic Training course, or only 50cc without doing the CBT. He's done his CBT.
However, nobody (so far) has said whether or not he can get the Residency Certificate in Krabi as that is the main sticking point, as far as I see it. He's more than happy to do all the tests (either at the government centre, driving school or Honda bike centre) if he can get the RC sorted