Please don't try to attribute Garry O'Neil's kicks to Muay Thai lol.
You might also like this KO at 1:59 then:
There used to be clinching back in the day, I believe they removed it from the allowed techniques as the bigger taller westerners would clinch and knee the smaller Japanese guys. From memory Dolph Lundgren for example would often grab the head of his smaller opponents and knee them. When he fought in the open weight world tournament in 1979 he won his first 2 fights by KO by grabbing the head of his opponent + knee.
I found a little clip here (not against a Japanese fighter though):
But yeah in Kyokushin Knockdown rules clinching or grabbing isn't allowed.
In some other organisations and styles they do allow grabbing, clinching and throws though.
For instance in Ashihara as you can see in this fight:
or even in Shidokan:
There is no reason not to and that's what I advised him in my post, he can use push kicks / mae geri's to keep his opponent at bay. Knees are fine too but if he's in range for knees his opponent might already be too close to him.