Focus on one sound at a time?

I'm not a professional, just a mom trying to help my son speak more clearly. This site has been great! My son (6 yrs old) can make all the phonetic sounds correctly but misuses many of them (velar fronting I believe). My question is this: should we be focusing on one sound at a time or multiple? For example should I only focus on his d/g sounds until we see progress and then move on to the t/k sounds and stay focused on those until he masters them and then move on etc? Or do I work on multiple sounds at a time? I find myself focusing on one area when we are doing lessons but then correcting him and practicing with him other sounds throughout the day. I just don't want to confuse him or overwhelm him-there are a lot to work on. Any thoughts would be great. Thank you in advance!