Extending AST to rolling 2 months notice

Tenancy Deposits

I have an AST with tenants due to expire in April. The tenant does not wish to sign up for another 6 months, as he says that he may be relocated with his job (true ??) and would like to go to a rolling 2 months notice either way. Does anyone have any ideas on how I can administer this ? I believe that if I do nothing that the AST will become a periodic tenancy with a months notice either way (as this is the period for rental payment), however is there any way I can ensure I get the two months notice the tenant is offering without a new AST (which he is unwilling to sign)?

He is a very good tenant (so far) and I prefer his proposal to his moving out, but do not know how I can set this up legally.

Many thanks.