No I wouldn't euthanise him! I only made the decision to euthanise the fish in one of my tanks because even though the velvet went away after treatment they were not eating, got extremely skinny and essentially just sat on the bottom of the tank not doing anything for three or so weeks.
With that said, my two year pair of wild bettas got velvet (their adult offspring were the ones who died) and after a successful treatment they have now given me around 20 more fry. So if by some chance your male does have velvet, be aware that is not necessarily a death sentence.
Also does the 'velvet' patch look like the yellow colouration on the head of my white CT male?
Having a yellow or orange looking discolouration on the top of the head like shown in the picture above, is perfectly normal.
Marbles are constantly changing colour. I have had blue and white bettas end up blue with red fins so just because a colour wasn't there originally doesn't mean it isn't going to crop up later.
Personally, I think you have nothing serious (at least in terms of velvet) to worry about. If you are concerned, just keep eyes on him and don't share any of the equipment that you use on his tank on any others you might have.