Thailand is a good gentle intro to Asia. The heat in Bangkok is insane though so giving yourself a few days once you arrive to acclimatize your bodies help. If I am honest, Thailand isn't my favorite country but its still a great country and the temples, beaches and activities in Krabi, pretty good. They have been in the tourism industry for years and it shows.
On the other hand I love Vietnam a LOT but its marmite. Some people come and hate it but I am in the love camp. Its crazy and insane but authentic and a country with immense history, culture and landscapes. Vietnam will assault each and every one of your sense at first. The constant honking, people touching your son's hair, your photo's being taken without anybody asking etc but you'll be deeply rewarded too.
My son loved Halong Bay a LOT because of the kayaking along the bay. We are in Da nang now, a coastal city 20km from Hoi An. Hoi An, itself is a nice little place but more a place that adults enjoy. We do have a theme park, games areas, skating etc that my son loves. If you have 6 weeks, you will find sometimes kids wont have the same enthusiasm for that charming village, another war museum etc so we mix the cultural stuff with regular stuff. Cinemas are a big thing here and we normally get releases weeks ahead of UK, something my son like boasting about.
If you decide you might want to add Vietnam, send me a PM. I can send you detailed information on what we do and even enjoy the 5 start resorts here as day guest and pricing. Your money will go far here, your flight will be the main cost. Eating out and things like massage, manicure and pedicure etc are pretty reasonable. All the best in your planning.