Is he nursing? Bottle? Is he curling his neck as he does this? Is it one leg at a time or leaning forward? How frequently, for how long and what does he do after stretch? Front legs & neck -- any difference than usual? Are his sides bulged at all? Have you seen him lay, chew cud, etc? How long has it been happening? Poop? Is he tender to touch/push on back, sides just below back, gut, especially just to rear of penis?
Don't know where are you located. What's your weather? Where has he been kept -- in, out, access at will, pastured, etc. Any new feed, hay?
Have you tried to assist -- syringed water, given anything, etc
Just asking to get some ideas of anything that may be affecting him to cause this. Nothing asked is more than inquiry & in no way is meant that anything should, could, help or not. Just not there to see the little one. or know more than this inquiry above.
Stretching can be an indication of stomach issues, as well as urinary issues which buckling seem to be prone to....I'm guessing that's probably why BeeKissed asked/suggested.