It's possible he isn't intentionally cutting you out but simply isn't actively keeping in touch (I often just don't think of contacting people), or is procrastinating about it for some reason (not sure when is a good time, has too much other stuff going on and isn't prepared for it, or is anxious about how you'll act when he does). Or possibly he just wants more independence and is avoiding contact because of it. Although it's possible he doesn't want contact with you, you shouldn't take that for granted just because he doesn't contact you.
He'll probably understand at least somewhat if you explain how you feel about it, but stay calm and don't pressure him. If he's not deliberately avoiding contact with you, it might work better if you periodically contact him rather than waiting for him to do it. You should ask him about it, but be sure to be calm and straightforward so it doesn't seem like an attack and so he feels he can respond honestly without upsetting you.
There are lots of reasons he might not keep in touch besides wanting to cut you off... for some of us, staying in touch, even with parents, isn't as easy as most people assume it should be -- but that doesn't always mean we don't want to stay in touch. I rarely call or visit my parents, but I still like to see them. If they leave it to me to initiate contact, I can go months without seeing them and only occasionally realize how long it's been... even when I do notice, I probably won't call them without a specific reason, unless I know they're expecting a call (at a specific time, not just "sometime soon"). Most of my family is like that to some extent -- we get along well when we see each other, but don't keep in touch.