Son is upset

Tomorrow I'm supposed to leave to pick my daughter up from her college and drive to my MILs house. My husband is taking my 15 year old and 4 year old sons to his mom's house. I'm making the drive because since it's in the opposite direction itself would make the total drive like 16 hours and it would be too much for my son. Like I said in my other post I decided to go because for the last few years my daughter has gone on the back burner because of my son going through intense trauma that's caused him to go into a long term age regressing. He acts like a toddler if a 15 year olds body and yes he's in therapy. It's hard because he was not like this before but that trauma he went through when he was 12 was too much for him to handle. He age regressed so much he isn't even potty trained anymore which is stressful and he's extremely attached to me. So dealing with all that. My daughter ended up on the back burner a lot because we were dealing with him but this is going to be my time to get one on one time with her and my husband said him and the boys will be fine. But my son just had a melt down until he fell asleep. He sleeps in the bed with us and he found out I'm not coming with them to Grandma's. I keep trying to explain I'm just getting his sister and then I'm meeting them there but he keeps asking why I'm leaving him. I've been explaining I'm not leaving him. I'll meet them there. So he started hugging me and saying he doesn't want me to go and I won't see him again and he cried for 2 hours straight. I got so overstimulated because I was also trying to get him ready for bed and it turned into a tantrum. He finally fell asleep. I'm wondering if this is a good idea and if my husband will actually be okay. He's confident in everything. But my son is very upset and not understand that I'm coming back and I'm not just leaving which he's scared of. I keep telling myself my daughter needs me too. We need this. I'm making the drive tomorrow. It will be 2 days at the MOST we are gone. My husband will be fine. He has everything he needs for both kids and he knows how to take care of our son. How to change him and how to handle his tantrums. It's harder because he's bigger and it will hurt more if he hits but I know my husband knows how to handle it. My husband is an amazing father to all 3 of our kids and I trust him. I know me and my daughter need this. So why do I feel so much guilt and this pit in my stomach? I didn't expect him to be thrilled by the news but seeing him so sad and confused hurt me. Don't worry. I'm not backing out on my daughter. I just want to make it easier for my son.