McChooky
Free Ranging
Legbars carry 2 copies of the blue eggs gene and pass it to all their chicks.You don't have to have Ameraucana's to get olive eggersI don't have much insight in those breeds as far as temperament, I only have 3 BCM's and 1 Belgium D'Uccle. 4 separate flocks.
I think I would try the Ameraucana and keep one of your Cream Legbars.
If your Ameraucana is pure and depending how dark your Marans and Welsummer eggs are you should hatch out Olive Eggers.
With all that said if this your first time with Roosters I would seriously think twice about letting your son take control of a Rooster flock. If he makes a wrong move there's a heavy chance of an injury. Roosters change your flock dynamics. He takes over your hens, there not yours or your sons anymore. Seriously read up on owning a Rooster and how to manage your flock with one or more. If you have had Roos then negate all that but I would still never let him near a Rooster without you personally covering him at least until he's old enough to understand a Roosters mindset and all the do's and don'ts and triggers especially in young cockerels.
Not trying to diswade you. Its a great thing he's doing being responsible and caring for living creatures on a daily basis. Lots of character building there but Roosters can be extremely dangerous to a young person who still thinks they can hug and play with their chickens.