I'm tempted to say red-bellied (Pallas') squirrel (Callosciurus erythraeus) due to the combination of red belly and greyish upperparts. I'm not sure what the white eye-ring is about though. Bangkok is out of range for them, but there's a big trade in pet squirrels in Thailand.
It may otherwise be a Finlayson's squirrel (Callosciurus finlaysonii), which is an extremely variable species, but I haven't seen any that look like that one I don't think.
this is indeed a red-bellied (Pallas') squirrel. I've never seen one with the white eye-ring before, but I was just at the zoo and these guys are everywhere. I only saw a couple of variable squirrels though. There are lots of red-bellied squirrels sold at Chatuchak market as pets, so I imagine that is where they started out from.