What is the best way to get my son to start opening up about his feelings more. He's 6 yrs old?

Be patient. A 6 yo is still in a world where imagination & play dominate their daily thoughts. Their brains do not process or "feel" the same intensity as adults nor are they able to elaborate them when they do. Docs often have kids this age draw pictures of life, to gain a window into their thoughts. Keep your ears open & ask open questions. If you show an interest, they may come back hours or days later & talk.