OP - you are extremely hard headed and it really isn't a trait to be proud of.
Of course there's nothing wrong with constructive criticism - but theres a time and a place and it simply wasnt necessary on this occasion. He's a child. & I can't fathom how you wilfully fail to see, that comparing your child to another child in a less favourable light, is WRONG. Its such an unpleasant thing to do
Your son and your DH's reaction to you, suggests your stubborn and critical traits are well known, your son is upset by it (hence his very strong reaction) and your DH is mightily sick of it
Its not ideal re the way your DH handled it - in front of your son. The conversation should have happened privately between you. But perhaps you feeling upset at being berated due to how you've made your son feel, may in turn give you an idea of how your son felt when you basically said "yeah its alright but your mate's is better, change it"
Its so transparently clear you don't like to feel that you're being criticised. Well if you can't take it, don't dish it out and then try to mask it as something else. You criticised your son, aimed to get him to do something in YOUR way - and you messed up. Period.
All this, over a toy made in the summer holidays. A fun activity, not an exam activity?!
*sigh