Yes, bless you for doing this. It makes my DS very happy when he is able to eat the same food as his friends.
The boy's mum is bound to have some ideas, and talking to her would be good. Don't be offended if she double checks what you have provided... I always double check, can't help myself!
Cheap biscuits are more likely to be dairy free (the pricey ones have butter, the value ones often just have veg. oil) but always check the packets, e.g. Co-op bourbons 'may contain milk' and are probably okay, but Asda smartprice have milk as a main ingredient and are definitely not okay.
Co-op breadsticks are dairy free, and some Tescos breadsticks. Houmous is a good idea, especially as many children love dipping things, salsa might be an option, and you can make an easy yet fairly respectable dairy-free "green dip" by mixing plain soya yoghurt with a decent quality mint sauce.
Jus-rol pastry is dairy free, including the vol-au-vent cases. It depends how much cooking you want to do!
If you're making celebration cakes, they work pretty well with Vitalite or Pure maragine. You can also use Vitalite or Pure to make 'buttercream' icing, and it's particularly good if you make it chocolately with cocoa powder as this hides the difference in taste. Nearly all cake decorations and glittery sprinkles etc. are dairy-free.
I'm glad that there are teachers like you who make such an effort to include all the children in these types of activities. Children like my son really appreciate you 
