Yet another cry for help about Fussy Eating Toddlers!?

My son is 14 months old and since we started weaning him, he's only ever eaten very fresh and even organic foods. He eats everything and I've been blessed with a very easy going baby when it comes to food. I'm a huge Annabel Karmel fan and I make everything for him from scratch. Up until about a month ago he was still eating everything. I'd like to add that recently he's also been staying with my in-laws on most weekends (they're leaving to Australia for good very soon so we want them to enjoy spending time with him as much as possible). My in-laws are career people and don't know how to cook so they give him all kinds of crisps, jar food, biscuits (Heinz chocolate flavoured ones!!!! Much to my constant disaproval!), etc... which he's not used to in our house. Now my son won't even eat apples, which used to be his favourite fruit. He's so picky with everything and won't even smell the food I offer him to know what it is. He simply turns his head away and starts screaming (a recent development in his behaviour as well!) I feel terrible and don't know how to get him to eat well again. I know I shouldn't worry as he's quite 'well-built' but I want him to get all his vitamins and even playing with food and making it fun doesn't work. Once he gets it in his head that he doesn't want the food, he automatically starts screaming. HELP?!