Tier-2 dependent child almost 18

I am in the UK on Tier-2 visa. My wife and son are abroad (Syria). My son will turn 18 on 4 April.
I want to apply for Tier-2 dependent visa for them very soon (before my son is 18). In the application form there is a question about the intended date of travel to the UK. Can we put that as June 2017 by that time my son will have turned 18 years?

I would be VERY surprised if they approved your son's visa.  But I honestly don't know for sure.  Is he still in fulltime education (high school)?

Yes he is still in full time school

Will he be when he moves to the UK though?  I know as Americans we have international schools in the UK, but we would be done by June after turning 18.  Will his international school study until a much later age?  If so, there *may* be a chance of approval.  But I'd recommend working with an attorney.  I don't think this will be easy!