Ancestry and the National Archives can be very helpful to you both; but it is according to if you are giving them information to work with. The more information you can give them. The more they can help you. There is Basic info if you have it. It will lead you directly to him and sometimes just give you a hint of him.
You basically need some of the info that I have mentioned in the following paragraphs; But just a portion of it might start you on your way. You have made family trees. Check the Public Family Trees to see if anyone else has made a family tree on him and has some military info; but check the below paragraphs.
What kind of information do you have in reference to his military history. The following should be supplied if you have it. (1.) First and Last Name and Mid Initial and any other christian name or names. Sometimes if his name is John. He might have been called Jack. (2.) ETB Estimated or definate date of birth. The year is generally sufficient (3.) What country did he enlist from United States, England, Canada, Austrailia, Ireland, Wales; etc. (4.) When did he enlist and where from. (5.) Was he in the Artillery, Infantry, etc. (6.)What was his Rank, Ser. or Regimental No., Batalion No. or Division. (7.) Regiment (8.) Medals. (9.) Date of Death (10.) Father, Mother, Spouse, Children (This helps in identifying him from his service and Pension records if there are any).
The more info you can give them. The more info they can give you. Its just like putting together a family tree.
In order for someone to help you search for him. They need certain info. Thats the name of the Game.