I am a step parent (though very very involved with my kids). They were 11, 8, and 6 when we told them I would be transitioning. They all handled it very very well and we have had no issues. In fact our oldest said "huh... Yeah that makes a lot of sense".
Kids usually have an easy time with it, especially young kids. Making the switch now eliminates a lot of the confusion with friends and other parents as your son probably hasn't really started school yet.
So, yes, I think it's easier for kids to understand as they do not have years of societal imprinting telling them what is "different".