Let me start to say that I am no expert on amphibians, it's one area I seriously need to improve on. Coming months I will be spending some time with one of Thailand's experts on amphibians, so hopefully that will improve my knowledge (or maybe I will only feel more lost...).
Anyway, my guess is Microhyla butleri. But I might be totally wrong.
There seems to be quite a lot of variation between specimens of the species within this genus. Which causes for confusion (at least for me, haha).
For ID-requests on frogs, best to post it in the Thai Herpetography group on facebook. There are a few guys on there who are like walking encyclopedias when it comes to amphibians and reptiles of Thailand.