My dog is weird with my bf

I've had my dog for a little over a year now. His previous owners left him at a PetSmart for boarding for about four months and never went back to get him. So he is a little bit nervous, but he is so sweet and silly, except with my boyfriend. He'll lay down with my boyfriend, sit on his lap, and play with my boyfriend but the second my boyfriend stands up my dog goes crazy. We've tried everything to mend their relationship lol