The 12 month SIM, in general, only offer 60 per month of 'any network' calls. That's 2 minutes per day. Though they do offer unlimited 'within network' calls. But having to determine on which network the person he is calling is on, is a complication he doesn't need.
There are a couple that offer unlimited 'any network' calls of up to 15 minutes per call. Can then hang up and call again.
https://www.alottechs.com/product/simtrue-maxspeed Gives you 60GB of data per month and unlimited free calls to any Thai number, for 2099 baht.
The cheapest with unlimited 'any network' calls https://www.alottechs.com/product/sim-angel Unlimited up to 15 Mbps Internet and unlimited 'any network' calls up to 15 minutes per call; 1499 baht. But I'm not sure about it's reliability. Some people complain about slow speeds.
If 60 minutes per month of 'any network' calls is enough, then he can choose practically any SIM package that suits. The cheapest being from DTAC, starting at 1490 baht
https://www.alottechs.com/product/sim-dtac-turbo-x100-select
https://www.alottechs.com/product/sim-dtac-infinity-x100-select