Go ahead and try it. You and your BF have tried the HONEST/straightforward/direct approach with your families. And his mom hasn’t budged on the issue. So, I guess you can try a more sneaky strategy. Who knows? It might work.
Let us know how it went. It could even be used as a suggestion for others in the same situation…IF…it turns out to be successful.
Do, pray regularly to Allah to make things easy for you.