Clarification on AST with our own son and his friend ?

Dear landlords,

We have a buy-to-let property with two decent-sized bedrooms in London where our son goes to University. He is currently living in the university’s hall of residence but would like to live and share with his friend in our own buy-to-let property from the next academic year onwards.

Can we ask the two young men to contract an Assured Shorthold Tenancy Agreement just like any other tenants? NB: both young men are financially supported by their parents, i.e. parents of both young men will be paying for their son’s respective (50-50) share of rent. We will be using our own savings to create our own taxable rental income (!).

The father of our son’s friend has written to me recently suggesting his son is a good friend of ours, surely both young men don’t have to contract an AST with me, instead, he suggests we just sign a simple agreement between parents based on agreed rent, as friends (we have met once briefly).

Please advise whether the proposed casual arrangement is sensible; is there any pitfall in the suggested casual arrangement? What would be the best way to go about letting the property to the young men to share together?

Thank you and looking forward to any comments/suggestions.

Best wishes,
XM