Short answer: The VA itself encourages it. VA care does not give a special enrollment period, so delaying Part B can mean a coverage gap and a lifelong late penalty if he later needs Medicare.
VA and Medicare do not coordinate: generally they cannot pay for the same service, and the veteran chooses which benefit to use each time. The VA pays for VA-authorized care; Medicare pays for Medicare-covered care elsewhere.
The VA’s own guidance encourages enrolling in Medicare as soon as eligible: Medicare covers non-VA hospitals and doctors, VA funding for lower priority groups could change, and a delayed Part B enrollment brings a penalty that grows each year and is paid for life. Medicare.gov notes that someone cannot sign up anytime just because VA services become unavailable.
Part D is different: VA drug coverage is creditable, so there is no Part D penalty if he enrolls when first eligible or within 63 days of losing VA or other creditable drug coverage. A Part D plan lets him fill prescriptions from non-VA doctors at a local pharmacy, but VA and Part D cannot be used for the same drug at the same time.