I swear my son would sleep in flannel/fleece pajamas every night if I would let him. He is perpetually cold, or at least he thinks he is! Our AC went out for a few days this summer and in temps of over 85 degrees INSIDE he still wanted to sleep in long pants and long sleeves!!
McCalls 5965 is my answer to that. I used a super lightweight knit that I purchased from Fabric.com a few weeks ago and made him some summertime pjs.
The neckband is a bit long in my opinion, and maybe even the neck itself a bit too wide. He is six, and I made these in a 4 and it falls off one shoulder on him! Also, there is an error on the leg band piece-it says the grain line should go one way, but in fact it should go the other in order for the cuffs to stretch to match up with the legs of the pants. Lesson learned. After cutting the bands out TWICE and realizing the error, I finally marked my pattern piece so next time I won't make the same mistake!