7 yr old acting out??

I have a 8 yr old daughter, a 7 yr old son, a 5 yr old son and another baby one the way. the problem is with my 7 yr old. In the last week he has started having bowel movements in his room and then hiding them. There have also been a few times that he has urinated in his room as well. His excuse is that he cannot make it to the bathroom in time. i say that if he has time to pull down his pants then he has time to make it to the bathroom. I am wondering if this is him acting out because of the new baby (which wont even be here for another 2 months). That however makes me wonder if this is just going to get worse after she arrives. What do I do? None of my children have ever acted out like this before!