our favourite is liquid silk - water based so it's toy safe
This is the search you need to do to find all the water-based ones Lovehoney sells
http://www.lovehoney.co.uk/sex/lubricants/water-based-lubes/
Well if you wanted to bring a little extra sensation perhaps this might help?
http://www.lovehoney.co.uk/product.cfm?p=4687&reviewid=20230#customer_reviews
Or if you want something the two of you can have a play with the toys and each other these might be a good starting point for you to figure out which suits you best?
https://www.lovehoney.co.uk/product.cfm?p=5349
You could also try using your bullet on him whilst you use his honeypot, perhaps you could blindfold him and tease him with different speeds of thrusting it, whilst straddling his legs and grinding yourself on him at the same time.
Let us know how the pair of you get on, I'm sure he will find it a turn on that you are taking an interest in his toys!
x
Liquid Silk based on the many reviews seems to be the best although I've not actually tried it yet. The 2% silicon content does however worry me on the safety of it's use with toys but in all likely hood it's probably dilute enough not to cause any damage.