Well ..
Little boys often dress up in mums clothes. She's often the one they spend time with most and they think she's fab so put on their heels etc. I mean that's common and standard.
Sneaking pairs of her knickers though? That sounds rather different. It's not dressing up as her in the same way. Would have to be surreptitious.
As a mum who used to have 6yo I'm surprised she didn't notice.
Bathtime?
Where did he put them after? Back where he got them?
I mean sounds like a difficult task to do covertly and often.
To feel close to distant mum. As pp said how many girls do this with their mums knickers?
Boys and girls with their dads pants?
His background sounds awful. Surely the answer should be unresolved issues. What's happening to make these feelings come back now? Getting support in getting to grips with that would be a better answer surely then yep.
Also it doesn't say if in relationship/ if was would they want to do this openly.
If it's literally just knickers under clothes then really it's not an issue surely.
Getting support with the rest of it is.