The answer I'm going to write is directly from WikiHow "How to ask someone if they're okay".
In the link, they assume you can talk in person. However, it doesn't seem to be the case for you. So, instead, I would strongly suggest calling and not using text. In a text, you don't ear the other person voice. Also, I have noticed that, for me, it's easier to say "I'm okay" via text than via voice call when I'm, in fact, not okay.
So here are the steps:
Preparing to talk
Have a private conversation. This means, if you call and they are in the middle of a crow, postpone the conversation for later. Your friend probably doesn't want other people to hear about his problems (I know I wouldn't).
Be sure to ask when your friend have time to talk. Interruptions aren't good for this kind of talk and having your mind somewhere else isn't good either. It's my experience that, in this kind of situation, people will likely dismiss you because they just don't have the time.
Call when you have time too. If you have something else in mind, your friend will probably notice and will be less likely to talk (that's, again, my experience).
Remember that you cannot "fix" someone's problems. If your friend isn't ready to talk, let it go (more about that later).
If talking about something personal make you nervous, you can write down some bullet points you want to address.
Addressing your concerns
Take a friendly yet concerned approach. When talking to your friend, be warm, open, and gentle. Show that you are concerned and want to help and support them. While you might choose to approach the conversation casually, make sure they know that you care.
- Say, “I’m concerned about you and want to know if you’re doing okay.”
Ask how they are. Once both of you appear ready to talk, begin asking some questions. You can start by simply asking, “Are you okay?”.
Keep in mind that there are lots of ways to see how your friend is doing. Ask, “How have you been lately?” You can also ask, “How are you doing? Do you want to talk?”
- Starting the conversation can be the hardest part. Jump right in and allow them to respond however they choose.
Mention something specific. If there’s something that worries you or concerns you, bring it up. Especially if your friend is surprised or somewhat defensive to your questions, expand a bit more. Talk about what you’ve noticed and why it concerns you.
- For example, say, “I’ve noticed you’re spending a lot of time alone lately. Are you doing okay?”
- You can also say, “You’ve been really secretive. Is there something going on?”
- Try to stick to objective observations without adding any assumptions or accusations.
In your case, you can say something like:
I have noticed that you have forgotten my birthday this year. I don't blame you for that but it made me worry that there was something going on.
Avoid confrontation. Notice if the person doesn’t want to talk about it or if they feel immediately defensive. You don’t want to cause a fight or argument. If the person is not responsive to your questions, drop it. Reiterate that you’re concerned and that you’re there for them.
- If the person is getting defensive, ask, “Is there someone else you’d like to talk to?” or, “I’ll leave you alone, but please don’t hesitate to call if you want to talk.”
- Understand that it might take a few conversations for them to open up about whatever’s going on. Try not to push the issue in your first conversation or two.
The next point in WikiHow is about suicide. Feel free to read it if you want but I don't think it's necessary to include it here. If someone diseagre with me, please leave a comment and I will include it.
In WikiHow, there is another part about "Responding to the problems". Since your question isn't about that, I won't write it down here.