If I were you, I have no objection for him to go out. (You should trust him before you have any evidence to suspect)
But I would say " Lo kung, lee tung kui hui my yea ho la, my yuen fai de fan loi ah, wo tung 2 kwo BB ho kwa gu lee ka, lee gi ng duck ka. Chung yau, lee kei ju da din wa be wo ah, BB sheung tung lee king kai ah, ho mun ah....."
Also, remind him (in sweet voice)NOT to pay for any of her purchases, save money for BB, they need money for milk powder and Univesity...then i think he understands what you mean.
When he comes back, you pretend bb voice, and say "Dad dad, lee yau mo my yea be untie ah, wo de lai fun lei" Hope he wont tell lies to kids.