Funny speech habit

[Fueledwithfairydustandgin]

My recently turned 4 yo DS is slightly delayed with his sound pronunciation and so has been receiving speech therapy at pre school. He has excellent listening and understanding and a way above average vocabulary apparently but there are a substantial number of letters he cannot say correctly. He has been improving and no one is concerned. I just wanted to give a bit of history.
About a week or 2 ago my MIL asked if I had noticed that DS was making a strange noise when he said s at the end of a sentence. I hadn't and spoke to DH who also hadn't noticed. We listened out for it and neither of us heard it at all. Until a couple of days ago where he started to make the noise when it was just me and him. It was exactly as my MIL said, at the end of sentences ending in S. however within 2 days he is doing it more and more frequently. It's now words ending in S,not just at the end of sentences and he's doing the noise for longer. It's like a sss noise, but because S is a letter he struggles with it's like tttsss ttttsssss ttttssssss
His speech therapist is on holiday (typical) and I'm a bit concerned at how quick it's escalating.
Any ideas? Advice?
I'm not making a deal of it at all with him but internally obviously a bit worried

I'm not a speech therapist, but when DS was struggling with s, the therapist encouraged him to try to say "ts" instead as it was easier for him to do and puts his mouth in the right position - and also is more understandable than a slushy s sound. So maybe he's learned it in therapy, or else he's naturally discovered a good way to approach the s sound?

But it's just the sudden strange overuse of it at the end of sentences that's worrying me a bit. I'm so nervous other kids are going to pick up on it and tease him. It is very noticeable. Much worse than his difficulty with certain letters