Well, I can't find a replacement offhand for a slim fast drink. Carnation instant breakfasts are gluten-free, but I haven't found yet whether they'd be dairy-free. I also don't know how the carbs would line up. (But then again, if your son's not drinking them, and doesn't want to drink them, you could just keep them for yourself.)
The snack could change to anything like carrot sticks, celery with peanut butter, pickles, coconut/soy milk yogurt.
I don't know what wasa crackers are, but
these are a good GF cracker substitute, then just make sure the lunchmeat is a gluten-free brand and use a dairy-free dressing (probably something mayo-based) instead of cheese.
Your second snack is already good if you just take away the cheese--substitute peanut butter for the apple, or coconut/soy yogurt. Instead of cheese puffcorn you could try
these.
Fish and watermelon are okay, and there are gluten-free tortillas. I can't remember the brand name right now, but they're rice-based.
I don't know about your particular brand of dark chocolate, but there are several dairy-free options for chocolate as long as it's not specifically "milk chocolate." And of course more fruit is always okay.