First of all, get over the guilt, feeling ashamed, etc. They are wasted emotions that get you nowhere good. You are where you are and it's OK. It really is OK to be human.
I alwasy tell people that the perfect PHYSICAL age to have children is 18, but the best EMOTIONAL age is 40. LOL
Being 40 is not being 70. My husband will be 43 in 2 months and I am 6 weeks pregnant with our 4th. When this baby is born, our kids will be 8, almost 5, and just 3. I try to look on the bright side - only TWO in booster seats that they can buckle themselves and a newborn....not 4 in carseats, like some of my friends have done!
I have to give you kudos for even thinking about your children and their well being. That shows me you are a responsible dad, but really, you need to put all your fears, guilt, and other emotions in a box and ship them off. You don't need them. What you need to do is be ok with where you are and be thankful that your little guy has such a great dad.
My husband is 5 years older than one of his sisters and 9 years older than his other sister. They are close and have great relationships, but that has to do with how the parents cultivate the relationships.
I have some friend who are 38 and 45 - they have 4 kids: 13, 10, 4, and 2. They are a super close family. The decided that the last lingering one needed a closer sibling, so they had another one...BUT that might not be what your family needs. DO NOT DO IT OUT OF GUILT. Your son will be fine either way....as long as he knows his parents love him.
A friend of mine was 48 and he died 3 weeks ago suddenly. He left behind a 17 and a 14 year old. Then again, my kids have 2 great grandmas who are 85 and 84 and both have all of their faculties together. One of them drives my son to soccer practice each week. You never know when it's your time and it doesn't matter. You have no control over the quntity here, but you DO have control over the QUALITY. It's the life in each moment. Don't fill these immediate moments with the worry about your future, things that will probably never happen.
Time is like a river.
You cannot touch the same water twice,
Because the flow that has passed will never pass again.
Enjoy every moment of this life.