That doesn't look like a snowboard boot to me, just a winter boot. You should get snowboad specific boots.
I'd try him with doubled up socks and see if a snowboard boot would fit.
My kid has big feet but is skinny, she had a burton boot last year with velcro strap at the top that even at its tightest wasnt all that tight and occasionally she would pop out of it. No harm no foul. What i should have done is add a bit more velcro to the strap so i could tighten it more.