Is the questioner sinful for thinking that an older person will not accept his advice, or does his behavior fall under the category of respecting elders?

Shyness towards an elder does not justify refraining from advising him. Advising with wisdom is an act of benevolence and fulfilling an obligatory right, just as Abraham advised his father and the Prophet advised Abu Talib. It is not a condition for the advised person to accept the advice for the advisor to fulfill his duty of enjoining good and forbidding evil. The aim is to clear one's conscience before Allah, as stated in the verse: {as an excuse to your Lord and perhaps they may become righteous} (7:164). In the case of sleeping on one's stomach, there is no sin upon you, because it is disliked (makruh) and not forbidden (haram). One of the conditions for the obligation of enjoining evil is a consensus on its prohibition or its strong likelihood.