My dad, a UK citezen, is planning a 98 day trip and also plans to visit INdonesia very near the end of the trip. We think, but this thread is asking for confirmation or help, that a 60day tourist visa, extended at immigration to 90 days, should be fine, then he can get a 15/30 day stamp on re-entry after being out of Thailand for about 4 days. Does this make sense, or will there be issues regarding more than 90 days within a certain period? In the past he has had non-imm O visas, but the cost is much higher for a short stay.

Thanks for your help, guys.

He will receive a 15 by land or 30 by air when he returns to Thailand.

it should be fine. There is no more rule about being max. 90 day in Thailand. The 15 or 30 days will depends on how he re-enters Thailand, by air it is 30 days by land 15 days.

Depending on his plan for going to Indonesia, a double entry tourist visa might also do.

Do the Thai part, fly to Indonesia via AirAsia (quite good). Get 30-day (only one available now) Indonesia visa on arrival (US$25, Euros OK). Fly back to Thai and get 30-days free on arrival.