He's 2.5yrs, he's been going since September and has a lot of friends in the group (from toddler groups etc). He goes 2 days a week.
When he first started, it was fine, he was excited about going. 2-3 weeks in, he started getting upset when I left him (but was fine literally up until I turned to walk out the door)
This has escalated to the point that this morning, he was screaming when I turned to leave 
The feedback I've had from the staff tells me that within 3 mins of me going, he is absolutely fine and has a whale of a time for the rest of the session (4hrs).
Likewise, when I pick him up, he spend most of the walk home telling me all about what he's done and who he's played with- all with a huge smile on his face!
What can I do to try and make the drop-off less traumatic for all concerned?