We had a similar issue with DS1 about then as well. At the same time we were also expecting another baby, so trying to get organised with strollers so we could have one in each car - we decided to get a little umbrella stroller to keep in DH's for if he took DS1 out- and we found that DS1 didn't seem to have a problem in that one- it is low to the ground so he can get in and out easily (though we do strap him in still, i think he felt a little more in control being down at his level).

As for the old stroller, we ended up selling it not long after and upgrading to a different one to hold both our boys and we've not had many issues with the new one (he did use it a few times on his own, but we mainly used the umbrella stroller before DS2 arrived)- though i think it helps now that DS1 realises DS2 is also in the stroller. So in our case here I think he found the new stroller a little more comfy. But i know this may not be your issue- just thought i'd share.

But i do recommend maybe trying an umbrella stroller- if you haven't tried or use one already (mum picked hers for $30, ours we paid about $100, cause we wanted it to lay down a little).