This time of year the water is nearing its warmest, and most people will feel comfortable in a sing-piece 7mm throughout the shallow side of Gilboa. A 5mm suit might suffice if you run warm or if you do most of your diving in the shallowest part of the shallow side. I know I couldn't do it, but I tend to get cold easily and dive dry at the quarry. But I recently saw a guy in a rashguard and boardshorts by the school busses!
If you intend on heading down to the tubes a lot (60'), then you might start to get a bit chilly in just a single-piece 7mm. But it depends on how much time you spend down there.
Give the quarry a call and ask if they have rental suits in your son's size.