I see!
I'd recommend using what you know of him for the tease side - personally, I'm a bum & leg man, so when the OH is teasing me with those, it's super effective. She also knows I like my nipples being pulled, so she might give them a gentle tweak, or move in as if to bite them and then only giv them a peck or something. I also like spanking, so she might give the impression she's going to and then just give a gentle slap.
In terms of more direct teasing - erection avoidance is a good one. Like running your hand down his chest, stomach and then just skirting over his cock, barely touching it. The same is true going upwards.
Build the tease up - it's not just physical, but psychological too.
Edging as described by mysteron is useful, but remember this is denial, so he mustn't be allowed to cum - or should he...?
In terms of applying that to an email, theres 2 ways to go, you can either write it out as if it was happening in a role play type thing
The second is you could take control, and tell him he's only allowed to do what you say... You could tell him no masturbating until you say so, then be descriptive of what type of self-pleasure you yourself might be undertaking.
It really is a personal thing, just try it and see where it goes.