I can't say just how much a lab should be eating .... but if its any help...
we have a rescue German Shepherd, came to us very underweight & with "issues" made worse by being in kennels (she refused to eat apparently)
We fed her up slowly but surely, with tinned food, mixer meal & complete biscuits over three meals a day (then two) & raw bones etc.
Having smaller but more frequent meals is definitely better for the digestion - Sheps can be prone to bloat & torsion, and the raw stuff helps "dry up" the poo (sorry!) but needs to be fed at a different time than cereal based stuff.
Now, Sheba has large bones as a complete meal, then tinned food & mixer as a separate meal. Complete biscuits tend to go right through her!
She has veg, the odd egg, and all the bits of the chickens we can't stomach!
Sheba will always, always eat more if she can. She doesn't "beg" like the old dog did, she sits beautifully and just waits, but having had the start in life that she did, we feel that she had to grab whatever food she could and so this has stayed with her.
we call her "dustbin dog" as she roots through the bins if she gets the chance, she fetches tins & packets,potatoes from the sack and even rifles my work bag to steal mints!
your rescue lab may well have deep-seated food issues, and so never be full. ANd as has been said, labs that aren't "working" are often greedy to the point of obesity.
I'd say split his meals... but don't feed too much more
any chance of posting phtotos, so we can see the two dogs and look at their apparent weight?? (and cos labs are lovely!)