5 y/o Wetting the bed

SO has 2 kids from his first marriage. His boys got potty trained before they were 2 or 3. I've been with him off and on for 4 years, and in that time his youngest has had 2, maybe 3 accidents in 4 years. He's peed the bed two days in a row now. We only get them Friday evening-Monday morning and they don't currently have a bedroom, so they sleep on the couch until we can move somewhere bigger. So, not only is he getting really embarrassed about peeing on his self, he's peeing all over my couch.  
Would there be a reason this his started happening ?? SO asked him if he had a bad dream or anything, he said no... My kid is only 18 months so I don't really have experience with this.

Have there been any big changes in routine, or lifestyle? Has there been any major changes at his mums place? My 6 year old is a bed wetter, so I know how frustrating it can get. My advise would be to invest in a good plastic mattress/couch protector and a brolly sheet. Or some special bed pants for him do he doesn't feel too embarrassed.

Maybe hes drinking too much before bed.... Or there's been some recent traumatic event that is causing him to regress?