Generally, planting tomatoes during or just before the rainy season is just asking for trouble. I have some cherry tomatoes under a roof, planted in bought in soil, but I'm not hoping for much. I have lovely soil but I, along with others in the village, am unable to successfully cultivate any members of the nightshade family, apart from chilli. Probably verticillium wilt, which BScTreelove will inevitably tell you is due to poor soil management, and then recommend some exotic product from the USA. You do realise that mentioning specific members will sort of annoy people like me?
Generally in Thailand, there isn't a sowing / planting season, it's a matter of are you willing to water when necessary, is for instance, planting watermelons just before the long, dry season starts, a good idea? Sorry DrTree.