It is very difficult to tell the temperament of a newly bought fish like a betta, I agree! You would expect them to be a bit stressed as well when you first get them.
I have tried a few times years ago to keep bette, and they never lived long or seemed as happy and interactive as the recent couple, and I believe that that is bcause I was just using dechlorinated tap water before, but this time am adding GH+ (to get TDS higher 240),occasionally KH+, and fulvic acid as I had these already for the shrimp tanks, together with the IAL. Both fish since doing this have been very active and healthy looking from day 1, so I have assumed it is down to JayC great parameter advice from setting up this tank.
I hope the salt helps and he recovers but once they get bad enough that you can really see they are sick I think it is probably too late usually to recover? Of coarse he may have already been sick when you got him as you haven't had him long?
You can use a frozen pea, mine are frozen, and my betta and killie had theirs this morning.
Do you know what the water parameters of his tank are, mine were PH7.5, tds 240, GH 8 drops, KH 4 drops last time I checked a few weeks ago?
I have my fingers crossed that he makes it...........
Simon