At 5mths old and 2 lbs he will not be able to keep his body heat for too long. For outdoor potty keep an area cleared of snow where he can go. Keep it as close to your back door as possible.
Booties are great especially with all the salt that gets thrown around. It will also insulate his feet from the cold.
So for you I suggest anything below 40 F be very cautious look for only 5 minute quick outings - see how he does. Then if you do want to walk him be prepared with a roomy Parka that you can pick him up and carry him inside very close to your body heat. I have a variety of outerwear gear - I will have to look up the names. I got Mukaluck booties (before I knew what great work All Dog Boots does. I have a fancy high tech fabric overcoat that does actually work to keep thd dog cool in summer and warm in winter. It is also rain proof as Toronto can get windy cold and rainey - then we can get snow sludge freezing rain - and temp swings of 50 degrees in one blinkedy day.
I also have a very very warm ;winter coat that I can layer under his high tech cover up.
But my advice for an active pup is to learn some indoor games to play with him. Try a non slip surface for fetch or retreive toy - yoga mats very cheap alternative to a rug. Create an obstacle course in your living room or bedroom - from what you have at home. Boxes - low stools - chairs - or go on an indulge in some low tech agility equipment. But remember no JUMPIN OFF Heights!! Teach sit and turn and lay down - Find the toy - that one is a blast for the pups.
In the wintertime when I walk with all my dogs - I make sure RAzzman can fit inside my outer winter jacket. My Blackies think -20F is ideal weather and will just lay out in the snow on a sunny day and bask in it. So to have a good walk in cold weather with the Yorkie he needs some warm up options.
In terms of the house; we set our temp at 68 and he is fine with that. How-ever maybe a sweater for your wee one would be in order. At night when we lower to 65 his crate is wool and silk lined and he sleeps very comfy. But he is now an adult and a 6lber.
It is so good you are planning now - as today we had our first snowflakes fall !! eeeck