The taxonomy of Asian Buteos is rather complicated currently and it is quite difficult to differentiate the exact Buteo species given the degree of overlap in their ranges and plumage differences. Based on range (HBW), Eastern Buzzard (Buteo japonicus) should be the most likely option in Thailand, although the paper below states that the Buteo japonicus from their native range in Japan look very different from the typical ones found in Thailand, which they propose are of a different subspecies under the Common Buzzard. Himalayan Buzzard is also possible (based on plumage) although HBW does state that Thailand is out of range. From what I've read in my attempts to identify the buzzards I saw in Thailand, there doesn't seem to be any consensus on the ranges of various species/subspecies and how they are split. Personally, I was unable to satisfactorily identify the buzzards I saw in Thailand myself. If you're interested, this is the paper that gives a good overview of the Buteo complex in Asia: http://www.caluta.liitin.net/Caluta/Caluta4.pdf.
But also, I'm not an expert on raptors and this is just my two cents based on my readings. Cheers!