If he is holding his arm out to keep you away, hit the elbow hard.
If he is just standing there you can step outside the arm and his body and head are open.
Just holding his arm out straight is not effective in keeping someone at distance, since the arm is stationary you can move it and move around it, just don't step inside where his other arm is ready to punch. If you step outside he can not throw the other hand.
Talk to different boxing trainers and ask for drills that help u cut off the distance. Anyway heres a vid of
Regilio Tuur (former SFW champ defended his title 6 times without ever losing it) getting inside on a taller fighter masterfully
You must be too stiff my friend. You're like a fat dog stuck at the end of a Animal contol officer's stick leash. You gotta shake my boy. Practice hugging people with your head on their left shoulder. Are you fighting someone too tall or big?
BOB and weave with short choppy steps.
Shout "look out behind you" and when he looks around sucker punch him.
come at him from angles
I want to apply pressure on my opponent but he stick his arms out straight so that I can't get close to him. What are my option in this situation if I want to get inside and hit him?