I would think the sling would be fine. Are you in contact with local babywearers for help trouble shooting?
You definitely don't want to a bjorn or some other front pack with a narrow seat for babies with hip issues, no.

Some kids don't like to be contained in particular ways, and you may need to try a few things. Plus, 3 times isn't all that many. My DS loved the hug a bub. my DD HATED it. She loved the ergo. I didn't. It wasn't good for her hips, anyway, due to her hypermobility - she needed more moulded support in a wrap or less-structured mei tai. Anyway, your DS's situation is different.