I just took this photo of my daughter's bum for you.

Think it has a bonus dog hair stuck to it, sorry!
Like you, as soon as I noticed this, I Googled and freaked myself out.
The good news is a lot of that information is misleading. At the time I Googled, my baby was about three months old and I was seeing a cranial osteopath because she was experiencing some breastfeeding problems. I asked her about it, in the vain hope she might know something, and she explained that there are three layers of cells that form from a fertilised egg, which become different parts of a foetus. The top layer becomes the skin, and sometimes, during the closing of the neural tube and the different parts of the baby's body, you can get a 'midline defect' on the back like this, as the skin gets pinched in.
Tongue ties are likely also a result of this, and are classed as a midline defect. A more serious example would be a cleft lip or palate. Interestingly, my daughter did have a posterior tongue tie, and a lip tie.
Anyway, she told me that she had seen hundreds, if not thousands, of babies whose backs looked like this and they had all been fine. She also told me her youngest daughter has this. And as it turns out, so does my 9-month-old niece! My best friend's son has a crooked top to his bottom, which I read could be a really bad sign - yet he is a perfectly normal three-year-old.
Your baby's spine and back will have been thoroughly checked at birth, during his midwife visit and at his six week check. Look in your red book for what they have written on the pages for these checks. There is a 'spine' option there.
It does look like he has two little sacral dimples there, which my LO does not have, but these are common and not a problem as long as they are shallow and you can see the bottom of them, or the bottom can be located by a medical professional. If you are still worried at all, they can easily put your mind at ease or start the ball rolling if there is any further investigation to be done.
Today my LO is a normally-developing 11-month-old who has recently taken her very first steps and is on her way to walking independently soon. Her bum 'v' is also much less obvious than it used to be as she has put on weight.
Hope this helps!