O.k
Carry on item: size of the bag is important. I wouldnt attempt to try to go beyond that size in case the overhead bin is full or the carry-on suitcase just doesnt fit. Most carry-on suitcases are compliant- just be careful if you have one that extends in size with an extra zipper etc. In terms of weight- its really hit or miss. Flying OUT of TLV, I find that they do weigh the carry on's but flying INTO TLV, its dependent on the agent. Worse case scenario, have a plastic bag available, but dont get too fussed over 1 pound.
http://www.ebags.com/product/skyway/...uctid=10172429 its an inexpensive carry on, that is also light.

In terms of the personal item- I don't know if they still allow for large personal items. Still, I bring a normal size school bag (which my laptop can fit into) and fill it to the max. Basically EL AL wont take a measuring tape, but ideally it should be able to fit underneath the seat in front of you (keep in mind the size of your son's legs- are they tall or short?). If he's lucky, he can stow it in the overhead as well. You mentioned that you have a generous backpack- if its one of those large Europe-bound bags it may not be a good idea unless its not stuffed - it needs to fit underneath a seat. It might be doable, but just dont stuff it to the max. I did it once, and there was barely any room for my feet, but I got my stuff there. Short term inconvenience for long term gain...kind've worth it .
In terms of weight, the bag shouldnt seem too heavy. I have only had my backpack weighed in TLV, and it was under the weight. But havent since.
If you are bringing a carry on and a personal item, I wouldn't push the weight on the personal item too far.

Checked luggage- I still use a camp duffel (no wheels) to bring stuff over, and its slightly over-sized in length, and no one has blinked. They mostly care about weight.