Of the bikes you list, the Trek seems like the best w/in your defined price range.  Though I would recommend the SL w/ its better component mix.   As far as your current ride goes, getting some thinner tires would probably help out a ton. Try something out in the 32 to 28 range and see what a difference that makes.

How old is your son, and how much does he like to ride?  My son just turned 14. Last year I picked him up an entry level road bike aluminum frame-carbon fork-Sora/Tiagra mix.  It made the difference for him and we were riding metric centuries by the end of the season.  If y'all are going to ride a lot I would say go for a relatively mid price road bike  (105/Ultegra mix) for yourself, and a lesser one for your son. It will be more enjoyable for everyone and you'll put down the miles because the bike rides nice and is really fun.   However, if all he is riding with you is the occasional 10-12 mile Saturday morning greenway ride, then the hybrid is probably adequate until he really begins to develop interest and desire to ride often and far.   More and more I find it is better to buy better than you think you need at the present time, rather than upgrade sooner than you wanted to.

Good luck, ride often, and have fun!  

sounds like a good plan.  the decreased rolling resistance with the skinny tires and higher pressure will make a big difference.  you will get more power from the new  pedal set- up too.  good luck w/ your son and his intro to riding. i know y'all will have a lot of fun and quality time together.

yes. the a520 is a road like pedal w/ same type of performance platform but uses quick release mountain bike cleats.  My guy at the LBS reccomended that pedal for my son because I still have him using MTB cleats.  The road spd "delta style" cleats are harder to get out of and I am just not going to make it that difficult for my son even though he is 14.  Nobody enjoys falling over cause you can't release your foot.  So, those cleats (520) are 1 sided entry w/ a thin/flat profile so you can ride into a turn and not hit the ground while pedaling.   The double sided 530s and M324 are big and chunky. I am sure they would work ok, but you ought to have a more aero lightweigt pedal.  With practice you can adjust to the 1 side entry without difficulty.  I may even buy myself a pair for my century rides since I am now using my son's old MTB shoes and they are better for walking than my road shoes. I still like the performance platform of the road cleats...just don't want to be walking in them too much especially at the SAGs or other tourist stops. I am guessing you are planning to run in the mtb shoes or do you have specific tri shoes that take the mtb cleats?