Hi! i have a 3 yr old rednose and he was the same way for a lil while. First thing is have PATIENCE!! You were right about telling him in a stern voice!!! Try not to yell or scream they react better to calm and stern.. Some ways i got him to understand when im telling him no is to make him first do the wrong action. For example: if he is tries to chew a shoe, First tell him “no” or “stop” and take the shoe away. Then put the shoe where he can reach it and allow him to attempt to chew it again. When he does say “no” or “stop” again and repeat until he stops trying to get it. this worked great for me i hope it helps you! Also sometimes using sounds to get his attention when he tries to do something wrong will help him learn whats wrong. For example: If he does something wrong that you want him to stop try to say “AY” to him or like a “Aat” sound. Dont yell it but say it a lil louder then your normal voice. It’ll grab his attention you then tell him “no” or” stop” and he’ll eventually understand.
Some good words to make commands:
“Come” if you say it and also make a come here hand motion itll help
“Stay” also do like a ‘hold on’ hand motion with it and that will help
when you are teching your commands dont always try to tech it wen he doing something wrong. At random moments of the day make him sit and stay then walk a way. if he stays make him come if he doesnt repeat until he does. When he gets it right make sure to let him know with a “good boy” and a short petting so he understnads that he did something right
Itll take a few months but keep practicing with him and he should get it in no time!! GOOD LUCK!! Hope this helps a lil bit!