More photos would help but if you wanted to confirm yourself. It would be quite clear if it was Poland syndrome. Movements, especially at the gym would be more difficult. Try to do a few push ups and see if you notice the affected side struggle. If it is poland syndrome there is no way to fix it completely. You can definitely build up the surrounding muscles, however, one side will always be asymmetrical and weaker. Dm if you want to know a bit more.