Worms or itching from stitches/shaved area

[mckenzie]

Old dog was castrated last Wednesday. He got an infection but that has cleared up and all seems fine.
Today however, he is agitated and sitting unusually, much further back with his legs sticking out at the front.
I'm thinking worms.
DH thinks itchy stitches or shaved area.
Your thoughts please.
I've tried to look for worms but dear dog is not playing ball

Not old dog, our dog.

If the stitches are still in they could be feeling tight or uncomfortable, making him sit in a different/odd way.

No, it's worms! I saw them today in his poo. Yuk!
I gave milbemax yesterday but he is still going mad, bless him. He is literally dragging his bum around everywhere, running in circles, jumping up.
How long until the tablet does its job do you know?
TIa