So get a reentry permit as I mentioned above. Once they approve you after a successful consular interview and give you back your passport you'll have 6 months to enter the US.
After arriving in the US, immediately send in your reentry permit application and wait about 3 weeks to complete the fingerprinting. Also initiate your son's green card application by filing I-130 for him.
After fingerprinting, you can return to the UK immediately and wait for the reentry permit to be issued. The permit will allow you to stay outside the US for 2 years without losing your green card. Hopefully by the end of those 2 years your son would have completed his own green card process. If necessary you can get another reentry permit.
That could cause another problem. When he's under 21 and unmarried he'll be in category F2A, which is for under-21 unmarried children of permanent residents. If you wait until he's 18 to initiate the process, that puts him at risk of turning 21 while waiting in the queue and aging out of F2A into F2B, which would add years to his waiting period.