There's nothing ergo-dynamic about it
That's unlikely to be because of the sling but because you are trying to do it in the most uncomfortable, body breaking position there is.
Try a side carry, as it's more comfortable for both of you, it enables them to snuggle in when overwhelmed, they can also sleep safely in it (they can't safely sleep forward facing) and whilst manufacturers recommend no more than 20-30 mins max forward facing (because of the above issues) there is no limit on side carrying.
When he's sightly older, you'll probably find it a lot more comfortable to have him on your back.
They other thing to think about is that if you are using a sling that is 10ish years old, especially for forward facing, it's likely to have a narrow base, which means it doesn't support their hips properly, and is also uncomfortable. If you do forward face, then it's important to have a wide based carrier which can properly support your babies hips.
Here are some random pics I've found on Google. Here's is a narrow, crotch dangling forward facing, and a more ergonomic (for them, not you!) one. Finally, I've put a side carrier for you to compare. Basically, carries will be most comfortable for you both if they mirror how babies like to be held.
Ps: go to your local sling library as you can try one or, have a proper fitting etc