Wow! 7 years. I could never go that long.
Definitely get the planning video. You have so much to catch up on.
To answer your question, No, you cannot use 2 days in 1 day. When you try to put the ticket into the turnstyle at the second park that day the computer will read that it's already been used that day and deny entry. You can only use the ticket at 1 park per day. You can leave a park and return to the same park in a day, but it really is a 1 park per day ticket. If you want to go to more than 1 park in a day then you must purchase the park hopper option.
Now for your son, this question has generated so much controversey on other websites.

Disney does not ask for proof of age for kids. They will take your word for how old you say your son is. Now, if he's around 6 feet tall and is sporting a 5 o'clock shadow,then yeah, they may ask for proof of age.
Even though you're not staying onsite, you can still take advantage of the Extra Magic Hours. When one park stays open late to Disney hotel guests. To get on rides, you would need to show your Disney hotel room key, but, to wander around, shop, and eat you can stay as long as you like. Disney won't ask you to leave. For example, at Epcot, World Showcase stays open on EMH nights. You could stay later and shop or eat if you wanted.
Ask any other questions you've got.
