I sure wouldn't do it. Not even with supervision. For a whole variety of reasons.
If you really really wanna anyhow, why not set up YOURSELF a separate email account, that he does not consider his and that he does not know the password to and that is treated as YOURS but he gets to sit on the chair next to you and use it for purposes that YOU specify.
Honestly though I think the internet is in too many ways a really bad influence on kids these days and while they are certainly bound to "find out about" and start using it sooner or later, why push for it to be sooner. At all in any way.
JMHO, good luck, have fun,
Pat