Yeah the female is smelling things on him that she does not like and it is exactly the same as coming home from the vet- they tend to react to the different smells. If they are smells of different cat or other things, your female could even not recognize him at some point and really screech at him.
Male and female cats together usually make a good pair b/c some males are threatened whan another male is added, but do not react the same way when a female is added to the household.
Letting your boy wander outside can be dangerous b/c male territory tends to be larger and he could be crossing streets while he is outside. Also that outside territory probably belongs to another cat who is outside more than your kitty is. If you plan on continuing to let him out, you should go with him and supervise b/c really, he is no more than a child out there.
Cats don't come with all the "outdoor" smarts people attribute to them, especially not cats who were raised indoors and have to deal with cats and other predators who were born and raised in the wild. If you don't plan to supervise your guy and you will still let him out, it would be wise on your part to at least follow him a few times so you can see his route. This is for when he doesn't come back one night and you need to go find him. If this happens, please do post on this forum b/c finding lost cats is very different than finding lost dogs.
I hope this helps!!