PM me. Live and work all over the place so got some useful info for you on VN, Cambodia and Laos. Thailand less so.
BTW Thailand most dangerous for riding around, they drive fast and some aggression, way way too many cars in the cities and not much space for bikes anymore. Net result, beware. VN is mostly extremely chill for driving and riding, apart from HCMC and where all the pro Putin Russian young lads are hanging out in Nha Trang and Quy Nhon, is very calm and low speed. But still, get insurance with expatriation for medical reasons (or death, not nice to think about, but its extremely expensive and extremely paperwork heavy to get a body of a loved one out of many countries in the world), and with adventure sports, motorbike cover (beware no UK license/CBT only does not constitute a licence in most SE asian countries so your insurance is void anyways). I would however recommend getting a scoot or better yet a Honda Wave/Blade which is a semi automatic and buy your own legit helmet (most of the worlds MC helmets are made there and are good quality, VN is very ANTI chinese fake trash products, so your usually OK buying from most places, I actually end up with more fake parts, components and clothing when buying in the UK than I do when in VN), knee pads, gloves and then just ride gentle, its safer than walking and I say that with a few years of experience in any given country.
Laos and Cambodia, I can give you some info that may be of use, some of its maybe not PC for public forums but will keep him safe against the forces at play in those areas.