We have a retractable stair gate which is much harder to open, you could try getting one of those?
You could just get a largeish padlock and put it around the bars possibly? Not ideal but would work and you could only put it on when you know he's got access to the gate
We took ours off a while back before she could walk down the stairs and consciously trained her how to shuffle down on her bum unaided for the first couple of weeks every time she went on them.. and she rarely even goes near them now because it’s not “off limits” anymore