We have a 4 years old nephew, he was diagnosed with astigmatism and strabism;what is the recommended treatment at his age and minimum age for surgery?

Depends. The timing and or use of surgery is usually based on the amount of success gained by patching.If the kid won't tolerate patching, glasses that blur the better eye are sometimes used.If patching corrects the strabismus, surgery is avoided. If alignment is bad for too long (years), any surgery would be cosmetic (alignment).I've seen it done on 2yo's.
Eye issues. Strabismus is when eye is either turning inwards or outward but they are not locked together in looking at something. Astigmatism is when eye is "tilted". You may mean he has some degree of amblyopia. That would be treated by "patching" the good eye and forcing the child to use the weaker eye.