Dog becoming more aggressive with age...

I have a question I'm hoping people on here will have experience with! Hopefully this was the right area to post!

We have an old puggle (11years old). Anyone who knows puggle personalities will know how stubborn and independent they can be. Our boy's name is Dunkin and he's been with our family his whole life. He has always been a curmudgeonly old man haha, but just kind of grumpy not mean.

As he is getting older, however, he is becoming much more reactive and aggressive in certain situations- mostly if we tell him "no" or try to get him away from something etc. in the past 3 weeks he has bitten myself and my sister. He didn't break skin or anything but it is still upsetting and we aren't quite sure what to do.

I think part of what makes him react like that is when we reach for his collar to pull him away from something (dropped food, dog poop, etc). I realize that this is not a good way to approach him as he is not comfortable with it, but sometimes that is just our instinct to get him out of the way.

We have 3 other male dogs in the house, but Dunkin is definitely the undisputed alpha- besides his problem does not seem to be with the other dogs but the people-

I have a baby coming in February so we are trying to make sure our home is safe for the baby.

Anyone have experience with something similar or have suggestions for how we can help correct this behavior? Getting rid of him is not an option. He is part of our family, but we do need to make sure that we have a safe environment for the baby to come home to come Feb. any suggestions would be appreciated!

I attached a picture so you can see him!