We had a fellow from Verdun Windows and Doors come in to have a look at Alex's window. He poked at it for a few minutes and then showed me where I could drive a screw to prevent the window from opening more than two inches.
Total cost of the operation: four dollars for a small container of screws and washers and some awkward time with the power drill and a screwdriver.
Wa-ay better than the $750 to replace the window we were quoted by another company. And certainly more helpful than the suggestion from another company that we should board up his window.
The alarm has proven impossible to seal but at least now we can rest knowing he can't get out. There is a risk in case of a fire but the fellow told me to get a note from my doctor explaining that the risk to Alex's safety in terms of falling outweighs his risk in terms of a fire. That should take care of things if, say, the fire department or Children's Aid has concerns.