Depending on his branch of service and other factors, his personal service record may have been destroyed in the fire at FPRC St. Louis in 1973. There is, however, an abundance of other sources from which key information regarding his service can be obtained. I don't do "how to's" and I don't give out sources that I still use. I charge a fee based on my costs.
If interested, send me a full name, date of birth, branch of service and a list of specific questions, that is, specific items of information regarding his service. I'll do a preliminary check and let you know exactly what the cost will be before you're committed to any agreement.
That's the best I can do (and the best you'll get) on this - or any - message board on the 'net.