I didn't have the Ergo, but there is no way Delilah would have tolerated her legs being froggied in any carrier at five months. She got ticked if I tried after 8 weeks!
I would try him with legs out and see how he goes. You can always try both ways to see which he would prefer, but I would guess he'll want his legs out at his age
My husband, who is a larger guy wears our daughter in the Ergo frequently, with her legs out. I noticed the other day, that when he wears her, her knees are higher and her bum is lower than when I wear her. Although it happened without us thinking about it, I think it prevents her legs from being spread too wide.
I hope this makes sense, I am not sure if I am describing it clearly.