[QtrBel]

With one I had that was similar when introducing new people I would have her behind a gate or fence so she could be loose and come up in her own time to investigate. She knew "cut it" meant either settle and behave or leave until you can act better.

 

[horselovinguy]

My friends have/had Border Collies....they were great watch dogs, affectionate to their humans, protective of them and their territory and their dogs were happiest when they had a job and constructive something to do.
My friends had cows and a pond of ducks who often were herded in the yard or field...
I remember their one dog was very "vocal"...he growled till told it was OK when someone approached...he did his job and did it well.
You did not sneak around their home, their property and barns without that dog telling someone there were others about..
🐴...

 

[FizAndAztec]

I have a border collie cross, and she is in no way a guard dog 😅

She can be timid though, and while she is very affectionate once she is comfortable, she definitely needs reassurance from me when meeting strangers. Working at a vet clinic, I have found this with most border collies.

Your boy is still new. Give him some time to settle in, and make sure to clearly establish yourself as his leader. This will help teach him to respect you when you put him in his place, as well as look to you for support when he's nervous or unsure.

I wouldn't be taking him to any situations that could be overwhelming just yet. Start with small walks around the block. Get him comfortable with cars, other people and their dogs passing by etc. Chastise him for reacting and reward him walking along calmly.

Still try to get him comfortable with your mum's dog, but don't build up to other dogs until he can pass then calmly in the street first.

One issue with him bonding with you so closely is of course jealousy and protectiveness. When you are with your mother's dog, make sure you pet them as well as him and get him used to the idea of sharing affection.

Best of luck and enjoy!