At this point the mc is 17 already, for most people that's nearing the end of their "growth spurts" anyway. While some guys do have a(nother) "late" one between 20-24 it is usually less pronounced in effect, not to mention if the mc does get to have one of those then it is likely Aeson or Saint or both will too. Since even if we select "tall" we're always notably shorter then those two I don't think the mc is ever going to catch up using natural means.
When it comes to the musculature while with the current rewrite it is already less dire then I once assumed. I imagine due to their enforced lifestyle that has not been conducive to keep up a gym membership they'd likely have been limited to bodyweight training and whatever exercise they naturally got. So the mc would have the build of someone who greatly prioritizes cardio. At best that looks something like this:
While nice and fit by the standards of most people it would still pale compared to the sculpted supermodel "Greek God" physiques of both of his "brothers".
As for steroids specifically. I already mentioned them being a shortcut, if Aeson and Saint were able to get where they are in terms of physique and musculature "naturally" it is highly plausible the mc could catch-up with 2-3 years of the same sort of diet and exercise schedule.
Possibly 1-2 years if we could get the sort of team top athletes have around them, coach, physician, nutritionist, etc (which we don't, so it's going to cost more time). Unfortunately time is one luxury our mc's do not get in this game, so when it comes to the actual gameplay I guess my mc is doomed to always look like the runt of the family.
The main other consideration here might be that:
a) Aeson does seem the type to use his "magic" to cheat a bit when it comes to enhancing his heroic stature. Which might make him less "natural" to begin with and unfortunately our mc's do not have anywhere near the control over and finesse with "magic" that Aeson has. So we couldn't ever hope to match him if that hypothesis of mine proves true.
b) The related issue that the full essence is likely going to turn the mc into a physical wreck even if that stupid demogod body does allow us to survive the process for more then a year.
c) Which might be counteracted with that "immortality" reward which would come I think at the cost of freezing our body, physique, musculature, height etc at the exact state they're in when we receive said "reward" which would be nice for Aeson and not so nice for my mc as it pretty much ensures he cannot change much physically anymore.