honestly if that was me in your shoes, (this may be disgusting)

but...

wank off a few times and store all you cum in a bottle or something for like a couple of days (without him knowing of course) if this is mess then buy your self a few condoms and indulge is some posh wanks (with the condom on) so i doesnt go every where, then when he's not there soak his toothbrush in it then rub it on rims of cups he uses and bottle, end of pens or what ever, just as long as you know it will go in his mouth, hahahahahahaha!

do it on the last day or something so you can get away with is dude, lemme know how you get on