In my opinion (and it's not a profesional one) I would get him off that shield asap! Even if you have to stay in bed with him the whole day and practice.
Same as with you, they gave me a shield at the hospital. My bub was prem and they told me it's better that way. After a month I brought her home and wanted to stop using the shield, but she would not drink from me directly. We tried for about three days. She just would not drink. She did not pick up weight quickly enough and my milk started to dry-up. I used to let her drink for about 30min per boob!! I was not getting enough stimulation and she was not getting enough milk. I tried expressing, but by that time I was too late. I tried all the remedies to get my milk back, but no sucess. After two months we switched to formula out of desperation and she thrived on that.
So if I were you and knew what I know now, I would teach him as soon as possible to cope without the shield.
HTH