You may wish to spend money consulting with another law firm. However, I can tell you for free that at the moment he has no need to worry, assuming that is he will easily qualify for Tier 2. But as I say, the longer it is left, the more chance there is that the Home office will curtail his leave. As long as this does not happen there is no issue with any 'gap' in residency, though he mustn't leave the country untilt his is sorted out, as he won't be able to get back in again.
We're not solicitors, so not in Chambers, but each of us have at least 10 years experience dealing directly with work permit issues.
Vicky