I’d go with the Kelty 50th Anniversary Pack ($240; www.kelty.com). This is a great external-frame pack that uses small-diameter tubing and carbon-fiber weight-bearing “springs” for great load transfer. Capacity is fine, with 5,900 cubic inches of space in the “regular” size, and lots of places to tie things. Plus, I don’t expect you’ll be doing much off-trail work with your young son, so you won’t rue the slightly lesser agility of an external-frame design. What you will appreciate is the greater back-cooling ability of the Kelty design. Overall, I think you’ll find it more comfortable than an internal-frame pack of similar size.
Once upon a time my second choice would have been the Dana Design Longbed ($290; www.danadesign.com). But, alas, that pack is no longer made, even though it might have been the best big-load pack ever produced. Certainly, Gregory’s Denali Pro ($460; www.gregorypacks.com) is a great pack, but it’s too technical for plain trail hiking. The Gregory Whitney ($329) has nearly identical suspension, 5,500 cubic inches of capacity in the regular size, and lots of nice features such as access zippers and adjustable hip belts and shoulder straps. It’s one of the best all-around backpacking packs out there. The Arc’Teryx Bora 95 ($395; www.arcteryx.com) would also work well.
So, first choice, Kelty. Second, the Whitney. Third, the Bora 95. Last but not least, go, hike, camp.