it is probably due to anxiety, if everyone is tense as you say then this will only make the problem worse. the best way to help him get over the anxiety is to be really calm and paitient about it. when he has an accident both your body language and what you are say are very important, you have to not mind at all and be very matter of fact, saying 'never mind next time you'll get to loo in time,' just help him clean up and dont mention it again. try not to talk about it to him, get annoyed or discuss it in front of him. basically not making any issue about it. if he does go to the loo then lots of praise and even a reward like a sticker for doing so. i hope this helps