If the saddle fits and it’s put in the right place before it’s snugged up the cinch shouldn’t need to be tighter.
If he’s using a thick pad the pad may be compressing after he gets on, in which case he should be able to gradually do that before mounting by taking up the cinch as he walks the horse around.
If that fails, maybe just plan on riding the horse for a few minutes and then getting off and snugging up the cinch, or you could take it up for him after he mounts.