Husband doesn't want to ask step son to babysit

Me and my husband tied the night last week. In two weeks we are going on our honeymoon. 4 kids together. 4 year old triplets and a 2 year old. My MIL agreed to watch the kids but she broke her hip. My step son just turned 18 and I asked my husband why we can't just ask him to watch the kids. My husband said he already knows he will say no because he's really busy. He's on his first year of college and taking 18 credit hours because he wants to graduate sooner and working part time and he has epilepsy so that worries him. My kids go to early head start until 2pm. He does a lot of online classes so he can still care for the kids and do school work. And my friend has epilepsy and is a mom. So I don't understand why he can't just watch the kids. My husband doesn't want to put that on him and thinks we should just postpone our honeymoon. I don't get why we should postpone instead of asking our adult step son to babysit