Hi, Thodoris,
The Miley actually reclines fully, meaning more than 150 degrees. That is the norm allowing to classify a stroller to be rśuitabel even for babies. So, at least from the lie-flat point of view, your son should nap just fine. I have actually 3 sons and the moment they reached about 1.5 years, they were comfortable napping even in a half-reclined position, so I wouldn't necessarily need a lie-flat stroller for an older taller toddler.
About the spaciousness/head-banging of your son, that could be a bit more tricky here. The Easywalker Miley is a ultracompact, which means a small, simple stroller that is not very roomy /more in an article HERE). The seatback here is not overly tall, nor is the legrest. I would NOT expect your son to nap comfortably at his 2.5 years/95 centimeters, he will be a bit cramped in there, unfortunately. For naps, you'd need a larger stroller/taller seat back.
My own advice is to try it out live if possible, OR get a stroller with a taller backrest (e.g., the Joolz Aer/Uppababy Minu that don't recline fully for real), OR get a bit larger pushchair (such as ABC Design Avus or similar mid-size lightweight) - the more so if comfortable naps are your priority. For those, you need space which these super small compact trollers simply don't offer.