My lad is six at the end of the month. He’s big for his age and would probably suit a bike for a 7-8 year old.

Any useable kids bikes out there where he could do some tame off roading with his old dad?

Something like this?

Isla bikes Beinn 20

Or this:-

Carrera Blast

Same dilemma last summer, we went for the gary fisher precaliber 20, he’s right behind me on the singletrack, sometimes in front. Very nice bike for the price.
precaliber

Just to add, it’s quite long so enough room to grow into.

My eldest loves his Beinn 20 small, but it it quite long. The large sounds ideal for your lad.