Mmm, I think the fact they are considering going to the ME so her boyfy can look for work makes it sound as if she wants to more permanent/longer than 6 months, doesn't it?
Has your boyfy thought about going for custody? Surely if the mum is as you say she is, there would be a chance he may get it?
I'm not sure you have to legally give back his passport but, as you say., there is the chance she may say it is lost but if you check the passport web pages, you may find that she needs to report a lost passport to the authorities - and I'm right, she must report it
http://www.ips.gov.uk/passport/lost.asp
The fact she must do this may put her off saying ti is lost?
There may be a way for you to contact the PP office to let them know she may do this? You can call them and see what they say?
Other than that consulting a solicitor is your best bet.