Your son would require a commercial ('buy-to-let) mortgage. Because it was defaults on such mortgages which largely led to the banking crisis, they're hard to get with large deposits (typically 30%) required, high rates of interest and evidence of substantial income needed.
Renting out properties involves meeting loads of legal requirements (e.g. placing deposits in a bonded scheme and getting safety checks carried out on gas and electrical appliances). Any profit made after making the mortgage payments (and necessary expenses) would be taxable. Landlords risk losing money during periods when they can't find a tenant.
You couldn't simply 'take over' your son's mortgage. You'd have to apply for a new mortgage; that could prove to be difficult until you'd been in your new employment for at least a year. (That, of course, assumes that you can actually find a regular job. I've not managed to during the past 7 years!).
Chris