D.B.
J.,
If you have already tried the suggestions others have written without success, then there is something else going on besides boredom and socialization issues.
No need to get excited and I'm not saying this to scare you, it is merely an avenue to investigate if the above does not do the trick. Too many times people, including medical professionals (...okay... especially medical professionals), don't take mom's questions regarding their children to seriously; they tend to blow it off as a "phase" or "nothing to worry about" or "they'll grow out of it.
You may need to look back through his 4 years of life to see if there was any significant event that may have caused this aversion (if it indeed is one). As you already know some events or issues may not seem significant to us or other people, but can have a whole different impact on someone else; especially considering their age, maturity level, state of mind, etc. Maybe there was a trauma of some kind, an event (like a dog/cat/animal making noise)that your child associates with the outdoors.
What was your child like as an infant, toddler? Were there feeding issues, tactile issues (being held, touching), noise level reactions, behaviors that warrent concern (head bagging, unusual body movements, arching of back, etc) slow developmentally in one area or another.
Sometimes very benign symptoms/reactions/behaviors can indicate
underlying problems.
Let me know what ends up working for you. Good Luck.
D. B.