Parenthood can put even a long term relationship on a different footing and sometimes it's tough to adjust.
How long were you a couple before the baby came along? Did you live together long before you were pregnant? (Sorry, 20 questions, trying to get a picture).
Was DP present at the birth, was he overwhelmed by seeing you in pain giving birth? He may have begun to feel anxious about hurting you unintentionally post-birth and the break in sexual intimacy may have been welcome. Then when you felt ready he was unable to get in the mood and he's got more anxious over time.
It's good he has broached the topic with you, he hasn't totally closed down.
Is he medically fit and well, any health worries, does he need a testosterone check? Suggest a trip to see his GP.
Even with the most willing partner it's possible to be genuinely tired a lot of the time with an infant in the home. Ordinarily I'd say you need to make more of an effort with each other, consciously making time to unwind and relax together. That means showing consideration, and giving each other your full attention at home no electronic distractions eg compulsively checking phones or tablets. Do the grandparents ever offer to have your LO for a few hours or overnight to give you both a break?
The wedding's planned for 2018. If you decided on the very morning you can't go through with it, that's your decision, no matter who pays for the venue, the food and drink etc.
I recommend counselling before this summer.