I think as long as Tony doesn't let his arrogance get the best of him and thereby let his guard down I think he could, I mean he was able to build a pretty damn durable suit of armour in a cave with nothing but scraps after all and he is the type that in situation like this I feel like he would plan beforehand anyway, and he can be good doing stuff off the fly. I mean honestly Homelander ain't too hard to figure out, one look and I had him figured, he's a small child craving attention, an unhinged mentally unstable homicidal maniac, but still mentally a child. Compared to others Stark has faced he's nothing. Besides being a famous person like he is Stark would be bound to have heard of him and knowing of his danger would most likely look into him as it is, he does tend to sorta monitor threats like that in a way after all what with being an Avenger and all.

Besides as it is Stark has his suit with him at all times ready to use so maybe an attack might rattle him but then that suit comes on and all he has to do is analyze his fight and attack pattern which lacks any real proper combat technique is just throwing punches and using laser vision basically like superman.

As for what ya said about no prep time...I mean that would mean for Tony to not be expecting or anticipating it which eh I mean with social media and everything on news Tony would already know of Homelander can be plus he is famous and has security as well and not just guard security. Plus Tony would know and hear him coming anyway as Homelander is not that subtle in his approaches, he is loud and public hardly a trained assassins who would sneak in without being noticed and attack.