If your son moved in and found a filthy oven quite soon after that, his first port of call should be his Landlord. Landlords are expected to deal with filthy ovens prior to re-renting out. Landlord should pay for a professional clean.
If the oven is past rescue/professional clean then the landlord should replace it.
Your son would do well to have a conversation with his landlord.
I am a landlord & would never ever expect a tenant to sort out a filthy oven unless it was themselves that caused it. My tenants are long term & in their rental agreement is a clause about keeping the oven (cavity) & hob in clean condition throughout the tenancy.
I've got another 'keen'... Ouch!