Ex won't stick to pickup schedule, kid is confused

This has been going on for 3 months now. We agreed 3pm pickups on Fridays but she's either 30-45 min late or doesn't show up until 4 when she feels like it. My son keeps asking if mommy is coming and I don't know what to tell him anymore. I've texted her multiple times being nice about it but nothing changes. Do I need to get a lawyer involved or is there something else I should try first? This is really affecting him.