My son is 4, 38lbs, and 41" tall, so I consider him slim.  I ordered him a 2/3T, which fit him PERFECT.  So if your little man is slim/avg, I would suggest one size down then they wear in pants for the perfect fit, or reg size to grow a bit.

My ONLY complaint is the elastic waistband.  My son has been potty trained for 2 1/2 years, but when he rushes out of the bathroom, the waistband tends to stay a tad rolled (don't ask how he accomplishes this, LOL).

It has not been easy to find underwear that fits him, NONE of the character boxers fit him, too saggy in the backside:(  He says he prefers the "Fruit of the Loom Boys 2-7 Toddler Assorted Color Boxer Brief 3-Pack", I also purchased them here on Amazon.