Under no circumstances should you put him on a regular mattress without some kind of safety frame. He's at an age where his development will happen in a blink, and what he couldn't do yesterday (ex: rollover), he'll be doing in the middle of the night, right off the bed or between the wall and mattress. You'll need your sleep- make sure he's safe so you can relax.
At 5 months old, he doesn't really "hate" the pack n play. I'm not sure what your attitude is about it, but babies in general will respond to a new situation by following your cues. The playpen will be your best friend over the next months, and if you plan to travel, you'll be so much happier if he can be safely, cleanly off the floor of rentals, hotels, etc. Maybe start placing him the pack n play more often, and letting him fuss a little. He WILL get used to it. Think of it as a safety issue: most kids don't love their carseat or stroller either, especially as they become more mobile.
I suspect that one of the problems is that you're uncomfortable putting him on used mattresses (I'm a total germaphobe myself). How about if you buy a high-quality piece of foam, cut it to fit the portacrib, pack-n-play or reg crib? You can cover it with a sheet, and then make sure the edges fit tightly, so he's safe.
My parents traveled with all 9 of us kids, and my DH and I have taken our 3 on road trips since they were born. The sooner your son learns to travel well, the more wonderful experiences you'll all have! Have fun!