Pasta meals or meals that have a high fat content do slow down the asorbtion rate of carbs, it's referred to has the pizza effect.. This is why chocolate isn't a good choice for treating an hypo, the fat content slows the adsortion right down..
As to injecting after a meal if you feel that the insulin is hitting the system before the food had a chance to adsorb, no this shouldn't be a problem you can then adjust to pitch the timing you start with giving his injection after he just finished eating and work from there..
There are pens that actually do a 0.5 unit, but hard to get hold off but worth nagging clinic for.. If you finding it very difficult to match insulin to carbs eaten, ask to be changed to an insulin pump which IMHO is one of the most effective ways of handling childrens diabetes, as it can be used both proactively and reactively, as it very difficult to keep up with the ever changing needs of children...
But I must say you seem to be getting to grips with it all, susing out the insulin timing against food adsorbtion doesn't come automatically to some...
What every you decided to do, please run it past your care team before doing so..