In all likelihood panties won't on their own do a particularly good job of hiding male genitalia. Your lover can look up "tucking" which is a technique typically involving pushing the testicles back up into the body cavity and then using surgical tape to tape back the penis creating a "smooth" front.
Personally I find that to be a little bit uncomfortable, but it definitely works.
Most of the time if I am just in and around the house I will just wear panties and not worry one bit about it (it's not like my partner doesn't know what is down there). I might tuck back a little but I won't go to great lengths.
Out and about I've found that a less severe tuck can be just fine as long as you wear some kind of restrictive shapewear (in my case, Spanx) which can keep things looking flat.
They also do make panties/gaffes that are designed to help with tucking, you can look into those as well, but personally I have never been too interested in going that route.
Hope that helps.
