partner finding out the sex without me

this is our second baby and we found out the sex with my daughter but this time I really want to wait to find out but my partner really wants to know at the 20 week scan, not sure if its a crazy idea but I'm thinking of letting him find out without me has anybody else done this? what are your opinions on it and if we do it should I make him keep it a secret from everyone or just me? I really want a surprise but I don't want to make him wait when he really wants to know