When your singular person could be male or female, you have three options: (1) use their, (2) use his/her, or (3) use his with a caveat.
Each team leader is responsible for their team.
(Using their for a singular is acceptable despite what some might tell you.)
Each team leader is responsible for his/her team.
(This is acceptable, but it's a bit clumsy.)
Each team leader is responsible for his team.
(This is definitely acceptable if all team leaders are male. It's even acceptable if they're not (with a caveat explaining that his means his/her), but it runs the risk of appearing sexist.)