This can only work if the stag is actually unconsciously drunk. If that happens and you have the necessaries, you plaster up his leg and tell him he fell over and you had to take him to hospital.
In theory he will believe you. Chances are he wil smell a rat unless you back it up with documents, referral to GP and assorted pills for pain in case he needs them.
You can choose to tell him before the wedding, after the wedding or whenver. Be careful: the bride will certainly have a sense of humour failure!
What you need
- Plaster of paris
- fake hospital documents