At 18 months, I would be tempted to give up the jars before you go - it won't be easy, but I can assure you that he won't allow himself to starve. Offer him proper food and milk and other drinks in normal quantities and he doesn't eat it take it away and give him his usual proportion of snacks and meals. I can guarantee that he will at least try some other food and he will probably eat at least one decent amount within 24 hours - irt sounds hard, but it is probably the only way that you will get hoim off the jars and it would be better to do it before you go.
If you really can t face it, try him on bags of dried cherios, fruit, raisins and cold puddings e.g. mullerrice or custard - at least they are more portable - savoury meals are more of a problem. You will also find some places like the family type of pubs that will warm them and also places like ikea if you are stuck.