Hi there and (((hugs))). Having mom and dad gone and come home with a new sibling is a huge change for a LO and even moreso when the LO is highly sensitive. My DD is on the sensitive side and at 6yrs she still wakes at night when something changes in her life. When DS1 was born, I think she woke for about 6 weeks and struggled to nap for awhile as well. When DS2 was born, she woke a handful of times. So, I'd say give it a couple months, as having your DH go back to work will be another change for him as well.
The best advice I can give for the IS is to pick a method and be consistent. You can use either WIWO or GW, whichever you think would work best.
Toddlers: Walk In/Walk Out vs. The Gradual Withdrawal Method (HOW TO CHOOSE) If possible, it would probably be best to settle the baby in a carrier or swing or something before you put him down for his nap, so you can spend some quality time with him and help him off to sleep. It's also important to try to get some one-on-one time with him during the day, so you can assure him of your love for him and his importance in the family. It's hard to do, but honestly the less you can tend to the baby, the better. Feed and settle the baby however possible and then try to make DS's life as much like it was before (within reason, of course). More (((hugs))), it will get better!