I have a picture of my kids father that was in the marines and I would love to make him a new uniform wearing his dress blues, would it be possible to put him in a dress blue marine uniform? Also if possible to flattened his tummy a bit too because he has lost a lot of weight due to cancer.
It really depends on your skills in Photoshop, your access to a photo of a uniform and/or head you can copy and blend into this picture, but unless the lighting and coloring and image quality are similar it will look very fake.
In technical terms, it is compositing several pictures together using layers and masks and free-transform to warp things into place. You can search for tutorials on head or body replacement and see how easy or hard it is to do.
If you give up on adding a uniform to this picture from another picture, making his body look thinner in this picture would be the lessen the shadow gradient on his stomach, perhaps using Dodge and Burn brushes, or the Shadows/Highlights function, after carefully masking only the part you want to change and giving the mask a wide feather so the transition between the untouched and manipulated parts isn't obvious. It also could end up looking somewhat fake.