One of my friends recently stopped talking to me as often as before because of his own personal struggles. How can I start conversations with him again?

I would say give your friend some space for a few days. This will let them process their thoughts and be mindfulwith what they may be struggling with. Then, when you think your friend has had some time for themselves, reach out to them just as many of the other times. Start the conversation casually, by asking them what have they been up to lately. Later on, after establishing conversation and if they start to open up about their personal struggles, just listen to them. You can also start the conversation by stating, "Hey I have not heard from you lately I wanted to know how you were doing."