Sand and cement coming off

[MollyImms]

Hello,
I recently had a new door installed and the installer used sand and cement around the frame to make good.
This was about 2 months ago. I painted it with a suitable external paint, but unfortunately the sand and cement is starting to crumble away in places.
The area where it's crumbling does take the brunt of the weather though.
Can anyone recommend a suitable product to repair it please?
Thanks so much.

[toolbox]

Call him back before you try anything else cement may no be the best way of sealing it is prone to cracking with the door being slammed even as a result of normal use.
A photo might help.