Weird Suckling Problem?

I have an adult neutered male (Phillip) and recently adopted a stray baby male (Allan). I don't know how old Allan is, but I don't think he's more than 4 months old. The two cats get along pretty well, but lately Allan has tried to suckle Phillip. I ignored it at first, thinking he was taken away from his mother too young, but then I noticed he had started suckling on Phillip's . Now he does it all the time, especially when they are sleeping. When I see it happening, I move him, and sometimes Phillip kicks him off, but why is he doing this? Should I keep moving him away from Phillip. or let it go