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Your son will need to register for self-assessment using form SA1 here. He will be issued with a tax return after the end of the tax year in which he should disclose all his taxable income. Take a look here for information on the types of expenses that can be claimed.
If your wife manages the letting, then there is no reason why she should not be paid but she will need to pay tax on it if she has income in excess of the personal allowance. Even if your wife would have no tax liability, she should still register as being self-employed with HMRC and compete tax returns. There are some notes and links to relevant forms here. If a self-employed individual has only one client, HMRC can say that they are an employee of that client and make your son have to operate a PAYE scheme as the employer but I'd be surprised if that happened in this situation.
I hope this helps but let me know if you have any further questions.