Your ex is entitled to wait 2 weeks before seeing his child again especially as school have sent your child home to isolate for 2 weeks.
Me personally as this is going to be happen very often soon if a child had tested positive i would still have contact but maybe would ask their mum if they were displaying symptoms and have them a different day say 48 hours later for example as they would be at home anyway.
It may be worth seeing as your child is off for 2 weeks that you let your ex know that your child is absolutely fine and if they would like to swap their days.So if they were due to have them this weekend perhaps they could have them during week or next weekend instead so everyones happy