Hi Kathleen! Your daughters have lovely names.
Here are some first name ideas for your son:
Your daughters have long, classic feminine names, so I tried to find some long, classic masculine names to match.
I also avoided J-names (don't want Margaret to be the odd one out).
I like most of the above with all of the middles you mentioned. I think the only combinations I would avoid are ones that give rise to repetitive initials (Dominic Donald) and ones that blend together because of like-sounds (TheodoreRobert).
Favorite combinations for me would be, let's see...Charles Walter and Sebastian Paul. Though I like a lot of others as well.