My daughter is now just 17 mths old with what sounds like a similar b/mark to your son. We started treatment at 6 mths old under GA. Claire has had 4 tx's todate, with lightening of her birthmark. Claire has the Candela V-beam used on her with over 150 zaps. She has never had any problems with GA or recovery of the laser. She has had 2 scars but they have disappeared shortly afterwards.
Your other issue is the glaucoma. Your son may be okay, like Claire, but our opthamologist still insists on 4 mthly checks up to 1yo, and we had another all clear last week with now 6 mthly checks. The other thing is if they are having GA in a hospital, an opthamologist may be able to check pressures while under GA and this will eleviate some trauma of having it done while awake. I haven't had it done myself, but a friend told me it is terrible thing to have happen, they are an adult.
Claire had an MRI at 5wks with the all clear, and our dermatologist (who seems to be on the ball about all b/mark issues) said this should be sufficient. Claire has not shown any developmental issues (actually the complete opposite - she doesn't stop) and has not had any seizures.
I hope this helps and you are able to push for early treatment. It is the best while the skin is so young, and not been exposed to the sun and all the harshness of the world.
All the best and if you would like to see photos you can email me email@example.com
Catherine (claire's mum - facial pws 17mo)