Hi jonesy,
Sorry to hear your son is having these problems.
To be honest there could be a few different explanations. It could be that the ratio is too high (i.e. he's taking too much insulin), which is causing the hypo; the later rise could be caused because the liver will release glucose when he goes hypo to try to combat it, and it often over-compensates.
Or it could be that the glucose is being released slowly and the insulin is acting quickly, as you suggest. The type of carbohydrate (high-GI vs. low-GI )and what it's eaten with can affect this massively (e.g. eating fat with any source of carbohydrate will slow down absorption, sometimes significantly).
Or it could simply be that you need to give the injection slightly later than you currently do, so the food has slightly longer to get into his system before the insulin kicks in.
Then of course, if your son is newly diagnosed he could still have some pancreatic function which can confound things - he might still be producing a little bit of his own insulin.
As you know, it's hard because we have a very "blunt tool" in injected insulin, and we're trying to use it to treat a very complex issue. Sadly it'll be experimentation that will ultimately give you the answers. One thing to consider is whether it might be easier with less carbs in his meals; not necessarily a low-carb diet, but just slightly reducing them to say a maximum of 50g per meal (or even lower). The reason I suggest this is that you're playing with smaller numbers; if he eats fewer carbs his blood sugar will change by less, and you'll inject less insulin which will also change his blood sugar less. Like I say I'm NOT suggesting a full-on low-carb diet, and it's probably important not to make too big a deal of it and turning meals into a huge issue; but reducing carb portions and upping protein and fat might help out.
Sorry I can't give any hard-and-fast answers, but I hope this helps give some ideas.
Cheers,
Nick.