Can't advise on leaving him with family. You know your son and family. But assuming you do go one problem might be getting a mobile phone signal to call home. Make sure that your phone is fully charged as getting re-charges can mean quite a queue.
As for making friends, no problem. Glasto is the friendliest festival I've ever been to. Say hallo to your neighbours, offer them a tea of coffee of something. You look out for their tent and they'll look out for yours.
Remember where your tent is relative to fixed landmarks like pylons, hedges and toilets. No good thinking it's next to a big blue one because when you return there will be twenty other big blue tents. Same with where you parked your car.
As for where to camp and what to do, read all the other Glastonbury threads on here. My general advice is camp closest to the area where you are likely to be at the end of the night so you don't have a long trek home when you're tired and emotional.
What to do day time? Loads. Bring decent walking boots and be prepared to walk a lot. I reckon around 12 miles a day as there is so much to see. In fact, there's so much that fitting the music in can be a challenge. Have fun.