N.B.
You can let him cry, but only 5 minutes before you go in the first time. (If you let him work himself up to the blood-curdling level, he can't settle himself back down.) After 5 minutes, go in, lay him down, shush him or tell him "time to go to sleep", rub his back a little, and walk out. When he fusses, give him 10 minutes, then repeat the process. Then go to 15 minutes. You want to teach him that you haven't abandoned him, but you're not letting him get up until he sleeps. If he's used to napping in the morning, maybe try an early lunch, and nap right after. Since this began with taking the side off his crib, you may want to rethink that for a little longer, or move him to the toddler bed with a rail. Maybe he doesn't feel safe.