How to explain the importance of pre-heating an oven?

For most baking, he's actually right, preheating is a waste of time and energy. The only time preheating matters is when you need to deliver the heat to the food more quickly for a better result, which is usually only for quick breads and yeast breads, where you get better rising action by putting the dough in a hot oven. For chicken and meats, preheating doesn't matter whatsoever. The fact that it's a convection oven makes up for the lack of preheating, since the heat is delivered to the food more effectively, which reduces cooking time. This has nothing to do with food safety. If the food is cooked to the proper internal temperature, it makes no difference how the heat was delivered, since the bacteria will be killed regardless. If anything, he may be able to reduce baking time just by using a meat thermometer to ensure that it's done instead of cooking it for so long. You might easily be able to convince him that he's just wasting electricity by cooking things longer than necessary, so using a thermometer could cut his energy costs quite a bit for cooking. Convection ovens usually take 25-30% less time for recipes that are timed for a conventional gas/electric oven.

So it appear that the old geezer is right and you're wrong.

The oven is heated differently while it is preheating, so food will not cook the same. I would recommend some kind of decoy activity (heating up a bagel, or a piece of pizza for example) before you are ready to actually start cooking. This way it would look like you are not preheating the oven, but then it will be all hot when you need to use it.

Hello Dan. I strongly recommend that you serve this man some food which was cooked in a non pre-heated oven. Ensure that he is aware of this fact. As he goes to take his first bite, slap him in the face and scream like a crazy forest wild man. Do this a few times and he will develop a strong phobia of food that was cooked in a non pre-heated oven. In effect, you will be subjecting him to Pavlovian conditioning. He will eventually fear food that wasn't cooked properly so much that he will even bake his Muller rice for half an hour, gas mark four.

Take the matter of how the food turned out aside from my opinion...

Preheating in general, takes the cold oven up to a preset temperature. To do this, more heat exertion is applied to the oven to bring up the temp in a reasonable amount of time. If you food is it the oven at this time, how certain are you with the cooking result?

Your FIL sounds like a crotchedy old bugger and I would not waste my breath trying to teach the stubborn fool anything. My advice is stay out of the kitchen or flick switches when the guy isn't looking or just save up and get the heck out of there...and take the dog with you.