I disagree somewhat with the other response. Your DS needs to be listened to and helped to navigate this problem.
When my DD was in Reception, she had a similar experience, with a girl that became quite obsessed with her, was a bit too physical, and didn't want her playing with other children. We kept up communication with the teacher, who was alert to the issue and was supportive of DD's need for some space (although she did tell us that DD was her own worst enemy at times, as she also gravitated towards this other child at times, despite coming home to us and asking us to 'rescue' her.)
We did some role play at home to help our DD to deal with the physical side of things, such as teaching her to say STOP very loudly with a backing off stance to make it clear to all around that she did not want this physical contact. That helped the teacher/TA to realise what was happening, and sent a clear message to the other child. DD did feel bad though, as she didn't want to get the other child into trouble.
We also encouraged DD to go to a playground/lunchtime supervisor to ask for 'space' from this other child. It didn't really help DD to play naturally with others, but did teach her that trusted adults would help her.
I should add that, just like your DS's clingy friend, DD's friend wasn't a horrible child at all - she was just working out social dynamics in her own way, but it was a bit too much for my DD at times. Thankfully, things improved as they matured.
I would definitely keep up dialogue with the teacher, and ask the teacher to agree some strategies to help your DS.