Oven cleaning with baking soda and vinegar

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.

If it's that dirty, I'd buy the stronger oven cleaning products or have it professionally cleaned to start with. It could take a very longer time with vinegar/baking soda.

I've used the white vinegar and baking soda method on the glass door of the oven and it was very effective used in conjunction with a ceramic hob scraper. I only left it about 5 minutes until it stopped fizzing. I used oven pride on the shelves etc. and found the longer I left them in the bag the more effective it was. I was cleaning the oven in my sons flat that had been rented out and the oven was only 18 months old but looked as if it hadn't been cleaned in years. Mine has never been in that state.

I agree about the professional clean. Just had my oven, hob and Cookerhood done - £65 - they look like new - well worth it !

Can’t beat the hired cleaner with his acid bath! It shifts everything.

I had it done about a year ago. It cost £45.

Might be best for this first clean, if the oven is really very bad, to get a professional oven cleaner in. They do not cost too much and leave it looking like new. After that a regular clean with the vinegar and soda every few months will keep it like that. Full instructions on this are available on u-tube.

I've not tried that method but I've had good results from the dishwasher tablet method. Bowl of warm water, a dishwasher tablet and rubber gloves. Dip the tablet in the water and rub it over the glass door, bottom and sides of the oven. Leave it to soak in for about half an hour and rinse off.