Morgage with old debts ??

Indout96

(Looks like I may have posted this in the wrong forum - so moved it fere)
Hi All
Around 7 years ago my son got into debt problems, he has since been dealing with a debt recovery agency who keep saying if you just pay off X then the slate will be clean, to date he has paid off two such amounts just for them to say he still owes more. (I have no idea how much he owed or has paid back but we are not talking massive dept)
He is now at the stage where he needs to get a mortgage. He is living with us at the moment together with his Fiance and 18 month old son but keeps telling us / fiance that he can not get a mortgage because of these old debts. Both he and Fiance work but her wage will not be enough for a mortgage on her own. (she has £20,000 deposit saved in her name)
Basically all I need to know is can his wage be taken into account on a mortgage or is he correct.
They are looking at renting but the rent means that the will not be able to save up for a house of their own so its catch 22