Hi,Rachel.
Congratulations on your pregnancy.
Your toddler can be as involved as you like with your pregnancy.Show him the scan pictures,talk to him about baby,there's lots of good toddler books about a new baby in the family.Do you have friends with a new baby,who your little one can meet.
Getting the nursery ready,getting baby clothes or anything like that your toddler can help o whatever level they can manage.My son helped pick his baby's mobile.He helped set up the cot and put the clothes in the drawer.
My daughter made a box for the baby(she was about 5).In the box she had a mobile she made herself,rattle,pictures,all things she made at school,but all things you could make at home with your son,when you're tired but looking for an activity to keep him busy.
All these things ,and many more help them prepare for the baby,so they're as involved as possible,they feel they're important in all this too.
Once baby gets here,make sure you have sit down activities,reading,a puzzle,even cbeebies,so while you feed baby your son can be part of it.Maybe have special toys just for that time.
When you're changing or bathing they can help out too.
I made a point of baby fitting into our routine as far as possible too.Baby can sleep in the pram while you play with your toddler in the park,they can sit in their little baby chair and watch the both of you play on the floor.
You'll still have evening and night feeds when it's just you and baby snuggled up for a feed,those are moments I still like to make the most of now,though my baby's coming to the end of all that now.
You've got loads of time to crack the potty training long before baby gets here,you don't want to be dealing with that too when you've a new baby.
Good luck with it all,and enjoy it.