S.,
From what you've shared, is that your son is confused and frustrated...and the only time he finds comfort is when your husband lays with him to nap.
At 18 months old, there are SO many things happening for a toddler. They do not just learn to sleep by themselves and they do not just understand that Mommy and the other kids are not doing things without them on purpose.
If he had trouble sleeping, when did this start? Just thoughts, but my son has always been a super light sleeper, and everything will catch his ear while he's sleeping. Have you tried white noise, or music to drown out the sounds of the house? My son always falls asleep with music and Mommy laying next him rubbing his back. I was able to transition him to sleep 2 hours on his own, after I left the bed at 18 months in a couple of months. Today at 2.5 he falls asleep with me sitting on the bed, and has the comfort knowing that he can call for me if he needs me or I will be sitting htere reading a book. Toddlers need peace of mind, and reassurance that their world is static and unchanging.
Your son is screaming because he is confused and frustrated...bottomline is he doesn't get it. He isn't sure what you want him to do. Do you have a naptime routine? Bedtime routine? These are key components to sending ques to kids that it's time to get some rest.
His age is prime for Seperation Anxiety, and with a new Baby in the house also the confusion over what has taken place to bring a new person into his world. This is a great age/stage to begin explaining details of the day. How things work, what Mommy does with the baby and how he can be your helper. My son experiencing Seperation Anxiety whenever change occurs that he is not prepared for or able to understand...like Mommy working late for a couple days in a row.
My friend's little boy (2) and baby girl (6mos.) are in daycare three days a week while she works part-time, and he refuses nap when he is home with her. He cries and shouts and yells, and even got to the point where he would throw things. Our Pedi told her that this was classic Seperation Anxiety and that at this point it was time to begin a new routine. She confessed to laying with him to get him to sleep and the doctor said, 'so?'.
It's not a bad thing to do whatever works for your child. If it helps and gets them through it's okay. It's just a matter of finding a way to make it work, and transition away from it at the right time for the child.
With the baby around he may be craving one-on-one time that he used to get before the little one arrived. It's really important and key to understand that his impulse control is not developed, and his ability to express himself and his emotions is at a base level of zero.
I think you may need to take a look at your day, and how it can best work around ALL your kiddos and create a calm environment for the toddler.
Best of luck!