Ok, If he is ex forces he would have received a 'reference' from his Boss as part of his dis-charge papers. Also, unless he is SF (and then you will face a brick wall of silence in response to any enquiries from his previous unit) you have every right to contact his last unit/Boss for a full reference. This should not 'piss him off' and you should inform him that you are doing this for completeness and not to check if he is a 'walt' or not.
The reality is that as an employer you should be aware of this - which leads me to question the authenticity of this post.......basics really.