Hi @ConfusedDadand welcome to the Airsoft asylum ?
it's good that you and your son are starting together.?
I would say read as much as poss to get an understanding of the basics. watch videos( check out our own @GAMBLEhe has a utube channel with tips and info) and bone up on the rules of your chosen site.
Spring chicken ? or not your gonna have to run till you drop! Lol.
No seriously you can take it at your own pace,play within your comfort zone and remember you are there for fun not punishment. I'm very late sixties now and I have times when I break into a sweat but most times I'm creeping around looking for glory lol.
£300-£400 is a good amount and you will get some good gear for that.Specna arms edge series, are in my opinion,a good bet as well as g&g stuff. Be guided a little bit by what you personally like the look of,after you will be using it.
It might be a good idea to play as a rental player for your first few games to get a feel for it and to decide if it's for you before spending out and also get to look at other people's equipment ( ooh matron).?
Make sure that you invest in good quality eyepro and nice sturdy boots with ankle support.gloves will protect fingers although direct hit will still be felt but not so bad.Lower face pro is advised but not compulsory unless player is under 18? Check with site.
Above all have fun( you are paying)
Listen to briefings,watch other people,ask questions,play within your limits,call your hits and have a great time?
Regards