Step son coming home smelling like wee

My step son is 12 and special needs. His mum wasn't around most of his life and just now started getting somewhat involved. He goes over her house one night a week but every time he comes home he smells like wee and his night clothes smelled as if he weed. His mum said he just pissed himself but he never does that over here. Every time I ask about it he just cries and rocks.