should my son in law be seeking a damp proofing specialist for repair on a damp extension wall, or another type of tradesman?

yes

post the job on here and the relevant tradesmen will call you.but try to explain in more detail eg damp inside outside mid wall,bottom and so on.you dont have to take the first tradesman that turns up,just take his advise,take his card make a note on it and get back to the one you believe to be the best,hope this helps

he need to get someone to check where the damp is coming from