Needs to be a stealth strike – after all, he does know where you live…
If it’s only a couple of times a fortnight, and a four pint bottle, then your retribution is probably going to be inflicted on an entire workplace, not just the guilty individual concerned.
(And the trouble with Picolax, of course, is getting the dose right – a cuppa’s worth of milk won’t have enough in it, and you can’t up the amount in case he drinks the lot!)