I have this harness: https://www.amazon.com/dp/B08F3QCV5P/ref=twister_B08F3RBNGX?_encoding=UTF8&psc=1 for my rowdy GSD mutt. There's many options for adjusting it to fit tight while still being loose enough not to hurt at all. Also, I suggest using a constricting collar for him. Like this: https://www.amazon.com/Hyhug-Premium-Upgraded-Anti-Escape-Martingale/dp/B08B7XBJBB/ref=sr_1_1_sspa?dchild=1&keywords=martingale+collar&qid=1612817013&sr=8-1-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUExTkxEMlJXMVJDVDVKJmVuY3J5cHRlZElkPUEwNjQzMjc3MzlENjgxS0NQR1o4MyZlbmNyeXB0ZWRBZElkPUEwNzE1MjUzMU83SFRSS0pBWkIxUCZ3aWRnZXROYW1lPXNwX2F0ZiZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU= and have a smaller leash attached to that, so even if he gets out of whatever harness he's in, he can't get out of the collar. My dog had problems with slipping out of collars too, but when I got a martingale, the trouble stopped.