potty training regression

my 4 year old step son has been potty trained for almost a year, recently he has started peeing his pants when he is at our house and only when he is at our house,he doesn't do it at his moms or any of his grandparents. we have tried talking to him about it, reminding him to go every 20 minutes to half hour, a potty chart and taking things as well as being grounded and nothing works. his mom is in the process of a divorce and I am currently pregnant and have been for six months, him peeing his pants has just started the last three months, we have him 2 weeks off 2 weeks on, both me and his dad are beyond frustrated and dont know what to do any ideas?