Why did other doctors tell me my infant has a sacral dimple and hes had an ultrasound,now they want an mri done but are telling me he needs surgery?

Depends: Your child might had developmental incomplete closure of the spine wrapping sheets, called spinal dysraphism, or spina bifida, which might have some connection with outside. Or the spinal cord end (resembling a horse tail) is too tight sitting inside and would stretch the spinal cord further as child is growing, which called tethered spinal cord syndrome. MRI would help to diagnose.