Only you can decide if it's worth it to restart or not. Comfort and closeness are valid (and lovely) aspects of breastfeeding too.
If it has been sore recently then it could be a latch problem and having a look at how you are holding him might cure it. Or might you be pregnant? It was sore for me to nurse my first when I was pregnant with no. 2.
As to it being possible to resume breastfeeding, it most likely would be. It's only been 3 days so not long really. I've heard of people only nursing on the weekends.
If it is something you'd like to do, then put him back to the breast and relax. There is information on kellymom.com about relactation that might be helpful.
DS was exclusively BF until 6 months. BF with solids until 10.5 months and then I started supplementing with formula along with solids. I still fed him too but this reduced to about 3 times a day. For about the last month I only fed him in the morning (hes not a good sleeper so wanted to night wean). Have found since I did this it's become increasingly sore to feed him and so for that reason in addition to going out for Hogmanay and him staying at my MILs overnight, I havent actually BF him for 3 days now.
I'm missing the closeness in the morning and whilst I know he isnt bothered at all during the day or evening, he still wants to feed in the morning and gets very upset when I dont feed him.
Can I restart? I do feel as though I have hardly any milk left and it was getting pretty painful. Is it worth trying or should I accept my Bfing days are over. He eats a lot of solids, cows milk etc so it would be more for closeness than nutrition I guess.