Use Legionary. Legionnaire is a different soldier from a much later period. That said, Roman troper's kept it simple. I'd go with variant human fighter with shield master. If he went to higher levels, I'd go with champion.
Battlemaster would be better imo
I second Champion for the rank and file Roman Legionary. At their peak they were remarkable for their ability to march all day then build a fortified camp for the night. An endurance excellently reflected by the Champions' Remarkable Athlete ability which gives them a bonus on Constitution checks. Sure, you could take proficiency in Athletics for a better bonus, but you only get two class proficiencies...
For officers, cavalry and auxilliary units, anything goes.