Yes, they do tend to sleep the way they did in the womb, but that's a very very odd position. Did you have a cesarean with a malpositioned baby? If not, then I can't imagine that he did sleep that way. I would talk to his pediatrician about the way he sleeps, it could indicate a problem.
I just did a google search on it and it is common with those infants who have sleep apnea or laryngomalacia, as it helps open the airway, so you may need a sleep study. Is he noisy when he sleeps? 60 to 70 % of down syndrome kids have obstructive sleep apnea and I found out about the apnea on a downs syndrome forum because many of those women have reported that their babies also sleep like yours does (and they also have apnea, or other such breathing issues at night.)
Low muscle tone is another thing that is linked to this sleeping position because the back is so much stronger than the core stomach muscles. The tone is not there in the stomach to even out the trunk, so the stronger set of muscles (the back) wins out. There are things that can be done for hypotonia, and ECI (early childhood intervention) can send you a therapist to the house for free or super cheap to help build muscle tone. Low muscle tone can lead to late milestones met, such as sitting up, rolling, crawling, walking, ect.
So you definitely need to talk to his dr and mention these possibilities (some pediatricians always say that everything is fine without further investigation.)
But don't worry about it, it might be nothing...and if is one of these things, they can be helped. It's just good to know if there IS something wrong.
S., mom to 4 princesses, CDA in infants and preschool, tons of experience working with babies!!!