depends on what attractions you're planning to do
with your
DAS, you'll need to walk up to the FP+ return line, and ask the greeter to get a return time.
This return time will be written by the Cast Member on your son's DAS card (which will also bear the number of guests allowed to ride with him)
At the time shown on your DAS you'll need to go back to the FP+ line and show the DAS card to the Cast Member, and then again to the second CM at the point where FP and standby line merge.
you will not be able to get another return time before you've ridden the ride for which you have a DAS return time.
based on july 10th this year (crowd 6 at MK)
peter pan's flight was at 55 minutes, so you'll get a return time at 45 minutes with the DAS.
This means that, you'll need to wait for 45 minutes (wherever you want, you could even get something to eat, do another ride (like teacups standby) or anything you like (even use a FP+ on another ride)
and you'll need to allow a few minutes to do the FP+ line, plus the ride time.
in the end, riding PPF during peak hour will take you a full hour, so unless you manage to ride something else during that "DAS wait time", you'll only do one ride in that hour
splash mountain was 75 minutes, which means a 65 minutes return time on the DAS.
winnie the pooh was 35 minutes, so 25 minutes on the DAS.
riding Buzz Lightyear would have taken 30 minutes wait (40 minutes standby) and the time to ride.
You'll get 3 FP+ to use during your day, choose them wisely, and that will help get 2 rides done in one hour (FP+ and DAS combined)
but in July, I would rather anticipate 2 rides every 90 minutes.
And it depends when you go. If you go on July 4th, some rides show 120 minutes, so when you get a DAS return time it will be for 110 minutes later, so unless you ride something standby (which might obviously cause an issue) you'll only get 1 ride in 2 hours (unless you insert a FP+ ride in between)
To achieve a better ride average, I would get there at park opening (9am) at a park on a non EMH day. That's when the crowds are lighter.
I'm sure I painted a very grim picture here, but I think it's rather realistic for an "average" day in July.
If you manage 2 rides per hour, that's a very good average. Don't expect to do more, be prepared for much less.
I would recommend you being at park opening and using morning Extra Magic Hours.