Frank,
We're trying to be helpful here & respond postively to your original cry for help.
You mention that accomodation is "not suitable", but obviously having 3 bedrooms IS suitable..... so we are merely suggesting that the Embassy has not received the paperwork that was expected by them.
You have to convince them that the accomodation IS suitable.
So, if you are renting, this means that you have to supply a copy of the tenancy agreement, it has to run for at least 6 more months, and you HAVE to include a letter from the landlord stating that he's OK with the "new" arrangement ie your son living there with his wife & child.
If its your own gaffe, you need to supply pictures (in & out), a council tax bill, and utility bills. If you bought the house recently, or had it up for sale, the estate agent's sales fly would be ideal "evidence".
Frank.... just follow the rules & you'll soon see your daughter-in-law & grandchild.
Without this, they are going to keep refusing you.