So glad you managed to catch him up in the sling
When we had preschool runs for DS we used to walk, the round trip would take me around an hour so DD would get a good nap in the sling and continue for a while after we got home - I never managed to get anything done with her in the sling though, not without waking her up, so I'd kind of sway and type away on here or on fb
We then made a habit of lunch in the park after morning preschool, so again DD would sleep there and DS would get lots of running around time whilst she slept!
I really think your system is working for you, so keep at it until/unless it becomes a problem. By all means keep working on those hammock naps, try to be as regular as you can with them so that he gets a chance to learn, and if you can gradually 'help' him a bit less that will make things easier for you in the long run. If you do have a few days with DH (or someone else) around to take care of DS1, you could have a serious attempt at ST at that time - I waited until my DH had a week off work when DD was 4.5mo, it was a good age for us actually.