I'd also strongly agree not to sit in the home end and support the away team.
My family is very eclectic and support a variety of teams. I've been to all the Premier League 'London' grounds. Without exception, the stewards turned back anyone (including children) wearing away kit from the home stands. It was pretty obvious if away supporters were in the home end once the first goal went in (either the only ones cheering or the only ones not!) The stewards got them out for their own safety. I think it's a mistake people only try once! Perhaps it's different outside London.
As a parent with a footie mad son (ironically we support Liverpool), I only ever got seats in the family stand at Anfield up until he was about 14. It's pretty well respected as a no-swearing zone. Definitely not like that everywhere else, a squaddie would blush!
I think the OP has already been given lots of great advice. My DS and I are Members (the annual gift is pretty good quality). Sign up to newsletters to make sure you're notified about ticket sales, especially for the summer friendly games. We got great seats at Wembley for a Liverpool vs Barcelona (when they were good) match a few years ago, which was superb.
The hospitality tickets are a fabulous treat if you can stretch to them. They have lots of different price ranges, the cheapest being the Sevens & Eights Lounges. The Boot Room restaurant is also really nice. You get the choice for pre or post match meal. Adult pays full price but it's much cheaper (£70 I think) for children. Since we travel up from the south, I've also got the hotel (Hilton) hospitality package. You get a room for the night, 3 course meal, match ticket & transport to and from Anfield included in the price. The Hotel is really well situated for exploring the city centre.
One other thing I'll mention - if your nephew needs to travel any kind of distance or a hotel stay is involved, Premier League matches can get cancelled/postponed at short notice. You'd be offered tickets for the new date, but might loose your travel/hotel costs unless you were insured.
I hope he has a great time!