I.W.
I'm sure my answer won't be a popular one. I think it's as clear as day....COMFORT HIM! I'm sorry, but I think your husband is being a tad mean. It's not wrong to keep trying to put him back in the bed. But I get the impression Dad's goal is to "make" him get over being scared rather than comfort him thru it. You said you feel pretty sure it's because of the fireworks and thunder. So there you go. There's your "why" answered and means this will just be a phase. I don't believe he's manipulating you. I believe he has a genuine need right now that needs to be met and if you meet that need now by showing him comfort and security, he'll feel safe and go back to sleeping on his own very soon.
If this were the middle of the day, and your son came to you saying he was scared, carry me, hold me, you would probably do just that. But "society" tells us when this same child has this same need at night, it's all of a sudden manipulation and you should not let him control the nighttime.
I can think of many times I needed to crawl in the bed with my parents. I can think of many times my own child has done that already. I'm so thankful my parents allowed it so it didn't seem foreign to me when my child needed it.