Immigrationboards.com

I have recently obtained the PR through the 10 years of legal residence, it's fine fore my wife, but my son has reach over the age of 18. But he has been in this country since 2003. I asked some lawyer and they have no idea

So I looked on the HO website and found this

http://www.ukba.homeoffice.gov.uk/visas ... ormset(f)/

Does he qualify for this application?

Has your son been in the UK legally for 10 years? If yes, he can apply for ILR.

Otherwise since both you and his mother are settled in UK, I guess this qualifies as an exceptional compassionate circumstance for applying to be your dependent (also if you can prove that you have been supporting him financially and he's still dependent on you, it would greatly improve his chances).

If there are exceptional compassionate circumstances, the following relatives who meet the above requirements may be allowed to join as settled person here:
- sons, daughters, sisters, brothers, uncles and aunts over the age of 18; and
- parents and grandparents under the age of 65.

From http://www.ukba.homeoffice.gov.uk/visas ... dependent/

Look at 319J of the immigration rules.
I assume you are applying on Set(O) in which case he should apply on a separate Set(O) with full application fee.He should also pass Life in UK test. Main point is prove he is completely dependent on you.