Pumping issues and supplementing

We are breastfeeding and giving my 7 week old formula because he was having trouble gaining weight. I’m not really sure why breastfeeding isn’t enough for him. I’ve done all the remedies. Basically my LC said he should get stronger and hopefully I can eventually stop the formula...thru also prescribed me a hospital grade pump because my pump just wasn’t producing enough...and yes I know he gets a good amount off me because I watch and hear him gulp the entire time, I leak, etc. basically if this pump doesn’t help I’m calling it quits. Has anyone else had a similar situation?