Hi all, I am at my wit's end & really need help...

Hi all, I am at my wit's end & really need help! I understand that toddlers are very fussy at times, but my 19month DS is driving me INSANE!! Today this is what he was offered: Breakfast - porridge with fruit & fruit toast Morning tea - homemade oat & fruit cookie Lunch - yogurt & peanut butter/honey sandwhich Afternoon tea - mcdonald's chocolate sundae & fruit bar Dinner - honey/mustard chicken with rice This is what he actually ate: Breakfast - literally a nibble of fruit toast Morning tea - half of the cookie Lunch - maybe half of the sandwhich (if that) Afternoon tea - 3 mouthfuls of sundae & the fruit bar Dinner - nothing What am I doing wrong??!! He's been like this for days now! How long can he keep going like this?? We have stopped offering second choices - he eats what we give him & make him sit at the table for all his meals with us. He has nothing else in between & the meals & snack times are kept very regular. If he finishes his dinner, he will get dessert ie. custard or icecream or yogurt, but he rarely gets this now. What else can I do?? He's a very slight boy - only weighing 9.6kgs at 18months. He's rarely sick & sleeps through, so he doesn't appear to be hungry, but I'm really starting to stress!! Any ideas?? Please?!