Hello again - I daresay this last reply of yours is likely to be the centre of the issue regarding his changed behaviour.
See some of Engineers previous replies. If it isn't clear - most adult males would consider it shameful to be beaten by the strength of a 12 year old boy 2/3 his size. Most other adult males would never let that guy forget it if they found out about it (the incident could quite seriously be brought up for years by others).
And not being his biological son - your husband doesn't have the out of boasting about his sons ever improving technique (other guys will jump on the truth of the matter rather quickly - usually completely ignoring anything to do with technique) "So your wife's boy beat you did he? How old did you say he was? And about so high, right <the other guy makes a motion shorter than your son actually is>? The story they then would tell to their colleagues would be hilarious, but not at all flattering to your husband.
Have you by chance been telling your friends the outcome? (I would very much hope not)
And in any case, while you say you aren't any less attracted to him (your original title does somewhat suggest otherwise by the way) - it is likely he sense of loss, combined with the pride you display in your eyes (for your son) around him (your husband)...leads him to believe he has truly lost status, not just in his own eyes, but in yours, and possibly in the eyes of your son.