I am not familiar with the items you mention and could find out nothing online. That 20 degree sleeping bag is probably more like a 30-35* bag--except for the really expensive brands, the ratings of most sleeping bags are at least 10 degrees off. How much does it weigh? You don't want to put more than about 10-12 lbs. on a 9-year-old kid; maybe less for his first trip. If it's a mummy bag and weighs less than 3 lbs., it's probably OK. It may last until your boy is 5'5", which is probably 5 years or so down the road. (Cheap sleeping bags tend to lose their loft faster.) It might be too big for him now, though, which means he'll have a lot of dead air space to warm with his body heat and therefore won't be as warm.

Does the pack really fit your boy? It's most important that a pack fit well and comfortably support 8-10 lbs inside. Be sure to get a pack with load lifters which will keep the straps from cutting into his shoulders (I found this out the hard way), and that will transfer nearly all the weight from the shoulder straps to the hip belt. I wouldn't try to put a small size adult pack on a child (I recently tried with disastrous results). A search of the Marmot website did not come up with this pack, so I couldn't find out anything about it. You might want to consider the REI Comet or Deuter Fox.

There are some things you don't want to buy too big and wait for the child to grow into them. Footwear is one of these things and a backpack is another.