Why dog doesn't show good body language when meeting other dogs?

Rocky our Staffie (don't judge) is showing behaviour which is of concern to me. His meetings towards owners is wonderful; happy, bouncy and very Staffie-like, however towards dogs it isn't like this. There are never fights because I avoid any meet and greets towards strange dogs - I have a feeling it's because of the sex of the dog, his unfriendliness is usually towards other male dogs. His friends who he loves; female Lab-cross, male Labradoodle, and two other females he barely plays with. We occasionally see a group of dogs in the mornings; male Jack Russell, male Staffie-cross and a male Bull Terrier cross (they don't get along due to alpha male disputes in the pack, I assume). So it's hard to understand what the instigator is. Today, I noticed his uneasy body language, whereby from a distance we saw a King Charles Spaniel (I believe it's a male because of it's size), both stood in ''that'' stance that dogs stand in when they see another, and as we got closer the other dog turned away to avoid the meeting. There is a problem, but I don't know what.