I am a cloth diaper user, so Pull Ups were never an option for me, nor did I want plastic pants - vinyl is terrible for babies' skin and doesn't allow it to breathe (not to mention all the toxins in it...)!
I used Potty Scotty undies. They are a bit looser than undies, which helped my son pull them up and down more easily on his own (the undies were really hard for him to pull up and down!). They also have an absorbent pad that catches accidents, but still allows them to feel wet. It is not an insert, however. Just an absorbent pad, but it did the job for MY heavy wetter. Potty Scotty also makes a version that has PUL (polyurethane laminate), which is waterproof but not vinyl. It is safe for babies, and breathable, and what the modern cloth diapers (like Fuzzi Bunz) are covered with. That would REALLY help your LO feel wet, yet not have any puddles on the floor or on clothes. We just used the regular Potty Scotty, though, and it was fine. http://www.pottyscotty.com/ and http://www.pottytrainingconcepts.com/
My other recommendation is a pattern called Ditto Daddy. IT's a pattern that you can buy to make boxers or briefs for your son (that look like Daddy's, hence the name!) and many WAHMs make them to sell. www.hyenacart.com is where many of the WAHMs sell them. you could do a search for ditto daddy to see. These have a special opening for inserts, which helps you adjust absorbency. that might be right up your alley. They are more expensive than the Potty Scotty, though.
Good luck!
**ETA here are a few that sell the Ditto Daddy undies:
http://hyenacart.com/DittoDaddyGear/
http://hyenacart.com/brandysboutique/
http://hyenacart.com/TheCoveredCaboose/
http://hyenacart.com/sudznseamz/
and here is the Ditto Daddy site where you can purchase the pattern (if you or someone you know can sew) http://hyenacart.com/DittoDaddy/