First thing I would recommend d is to watch this video with him:
http://forum.simplydiscus.com/showth...TCH-THIS-VIDEO!
Are you cycling the tank with pure ammonia? And if you aren't sure what I mean I would recommend to research how to "cycle a tank with ammonia" - there should be lots of posts about this on Simply.
I am very hesitant about your plan... I've been fishkeeping since I was 12 and these were the hardest fish for me to keep. There are lots of concerns raised from your plan... 30% w/c with such small discus, not aging the water in a barrel before water changes being two big ones. The video will help get some base info, and then read thru the stickies in the beginner section also.
Have you ph tested your aged water? This is where you test the ph of your tap water immediately from the tap, then fill a bucket with the same tap water and aerate it with an air stone for a day, and retest it after aeration to see if the ph has changed - it is important to know if the ph changes because this will help determine if you need to age your water in a barrel before water changes.
Hans is great, I would buy from him myself - any of the sponsors here at Simply are good choices to buy from. Hans is a sponsor. I personally don't know about live aquaria so I can't comment on them.
2 Heaters is fine - I would recommend inkbird heater controllers to prevent the tank from overheating if one of them breaks and sticks in the ON position.
I wouldnt recommend to mix angels. Some people do, but it is riskier than just going with discus because they are more aggressive feeders and I have found they bullied my discus when I tried it myself. I love a tank of angels, which are much hardier than discus and easier to keep... I would lean towards a tank of just angels for your son if he had to choose between the two species... esp because you are mentioning time commitment concerns... angels would be much more forgiving on the water-changing front.
There is more I have to add but I have company coming, there will def be others with more advice as well, but I'll try to follow up on the post later on to make sure we covered your questions. It is great to hear he is so interested in the hobby at a young age!! That's when I started too, and having a supportive parent like yourself makes a world of difference!! Big hugs to my mom and all the parents out there for letting us curious kids explore hobbies and fill the house with fish tanks lol <3