HELP!! Aggressive Aussie

Hi Reddit,

My wife and I adopted a sweet Aussie puppy back in May from the shelter. He's now 9 months old and up until about 3 weeks ago was the sweetest puppy and had his normal Aussie antics/misbehavings, but nothing serious.

Recently and seemingly of nowhere he's started to aggressively resource guard. He'd growl, bark and has recently started showing teeth and today, bit me drawing blood. What's worse is his aggressive behavior is carrying over even when no food is involved, again biting/attacking. He will snap and bark 50% of the time when trying to take his leash on and off to go outside.

I have a vet appointment next week to see if there's something physically going on making him like this all of the sudden and also am looking at finding a behavioralist soon too. He has not been neutered yet either (in case that info is helpful).

I'm just concerned there's something physically going on with him and he's lashing out all of the sudden is because of a health issue.

I work for home and take him on 3 mile walks twice per day but we don't have a yard that he can necessarily run in at his leisure.

Thanks for any advice.