I saw that guy on Aquabid and was hoping someone from the forum would get him so I had a chance to see if it was genetic
If he's the only one out of the spawn with big pecs, then no, you won't have a better chance getting that trait by breeding him back to his mother because it is most likely a random mutation and
neither of his parents carry the gene at all. The good news is that if this is the case, it is also probably a dominant trait, so it should show up in about 50% of his offspring.
I've also got a betta with huge pectorals... I had two, but Bartleby became ill while I was on vacation and because I wasn't there to care for him he later died. Meet Serafino