Hi B. -
We had 3 children in under 3 years and never "baby proofed" anything in our 2 story house. When the baby started going up the stairs, we showed her how to do it safely and then set the limit that she had to get a grown up to do it with her. We put the trash in the cabinet and gave her trash to throw away so she understood that it had a purpose. We kept lids down on the toilet and made the bathroom a no play zone. We also put their cups, bowls, snacks and milk at their level so that they could help themselves. We went with the idea that not everyone had a child proof house and we wanted our children to be able to go into any place and be respectful of the boundaries. I had so many friend that locked everything and put everything up and those kids couldn't be taken anywhere because they would touch everything in other people's homes.
I hope this doesn't come off as harsh but, this is the time to start with discipline by setting limits and boundaries. Children don't learn by being gated and locked out of places. They need to know that they have parents that will tell them what is acceptable and that there are consequences for doing things that are unacceptable. Yes, even at 15 months. It was definitely more work to be everywhere my child was in order to keep them safe at different stages, but it has absolutely paid off. They are self sufficient, well mannered, inquisitive, and fun loving 3, 5, and 6 year olds that can go anywhere with out worry.
Good luck to you, keep loving Isaac. Everyday is a gift to be treasured!