M.O.
M.,
I'll start off by saying that I am a mom of two girls - so I don't have experience with the boy angle...
I don't believe in holding children back UNLESS there is an educational reason to do so, such as a premie, learning disability, etc. I don't think that an Aril birthday is late. A July or August, yes, but April is very much in the range of "normal" for his peers.
Of course, ask his teachers how he is. It's only March now, so you have 5 months to "get him ready" for whatever new adventures Kindergarten holds.
Encourage him to act his age. If he acts like a 2 yr old, treat him like one! No picking his own snacks. Naps. Losing simple privileges HE gets that his younger sibling doesn't. Once he realizes that it's not COOL to be a "baby" then you can really start treating him like "a big boy".
Start by laying his clothes out the night before, encourage him to get dressed while you do, buy him a "new" tooth brush/tooth paste so he can "do it himself" and then you check his work.
If he regresses, here and there, remind him of what privileges he will loose. This worked GREAT with my older daughter when she "regressed" because she thought being a baby was cooler, got more attention, etc.
I don't doubt for one second that boys and girls have different developmental levels at different ages. I don't doubt that girls seem to have an ability to sit still, have better fine motor (meaning printing skills) and interest in arts and crafts...boys appear to be more physical (meaning rough housing/running around), more interested in building/cars/how things work and more even keeled emotions. I don't think that justifies keeping kids out of school for the grade they are chronologically ready for. I think teachers are VERY aware that different kids have different needs and different learning styles. Kindergarten is a BIG year of change for all kids. Your son will be fine. Just start expecting more from him now.
I hope this helps!