My5 I am so sorry to read about your precious son.
I lost my son 14 years ago to SIDS and I lost my daughter 5 years ago to a genetic illness.
You are still in a haze just now, for me, after I lost my son it took nearly a year to properly sink in, and in the very early days when there were still things to do (deciding what to do with his things, sorting clothes and photos etc) it didn't seem real.
You can't even take each day at a time at the moment, you need to take each minute at a time at this stage.
Stop torturing yourself that he was struggling to breathe, he wasn't, he was sleeping and simply forgot to breathe, no pain, no suffering or struggling.
When my son died he was my only child, when my daughter died she had 2 siblings, I read them a story called 'The Waterbug' (you can find it online) to help them understand in an age appropriate way (they were 2 and 5 at the time) why their sister couldn't come back, although I had already told them about their brother so they had a little understanding.
Don't stop speaking about him and keep lots of photos around. My kids always include their brother and sister when they speak about family I find that helps me as well as them.
Don't make any hasty decisions about what to do with his things, if you find it too upsetting to have them around just now see if a family member will store them for you so you can make that choice with a clear head.
Also, and I don't mean to upset you, you need to have a little think about what you will say when people (as they all do) ask questions about your family. I never considered that type of thing when I lost my son and would stand there not knowing what to say and found it really upsetting. Its better to have an answer ready so you don't have to explain to the woman packing your shopping or the lady at the hairdressers and upset yourself further. (This is why I never ask people how many children they have now). I really hope it was ok to say that to you its just one of the things I wish I had thought about.
Most of all be gentle on yourself, it wasn't your fault, there was nothing you could have done, SIDS is totally random. Cry when you need to cry, talk when you need to talk, and include the kids in it too, they will take their lead from you. Dexter is still a huge part of your family and always will be.
Much love to you and your family xxxxxxxx