Someone begins attending a congregation but does not ask to be a member there. He starts forsaking the assembly. Can you withdraw from him? He was baptized.
Before I answer the question, I want to note that some congregations add people to their membership simply because they keep attending services at the local church. They might not be aware that at other congregations you must ask to be a member. Did someone approached them about becoming part of the church? If not, then it is a short coming on the congregation's side for not making it clear whether they are committing to this church or just feeling out the congregation for a time.
I am going on the assumption the congregation has done its due diligence, the person was asked to become a member, and the response was "no," he does not want to be a member. If this is the case, then local congregation does not have oversight of this Christian. Thus, the church cannot withdraw if the person is not a part of the local congregation and has stopped coming. The brother has chosen to withdraw himself and not any other action can be done in "disciplinary" action beyond that. The Christian has done most of the work already cutting himself off.
Besides, withdrawal is not what God calls us to do as first step when someone is straying. There are others steps we do first. Below are some suggestions what to do next.
Someone need to take time to investigate the issue directly with this person. Only then will you know what steps need to be taken next. Try calling the person and seeing if they are having any issues (health, family, depression, sinning, loss of faith, jobless, new job, moved, going to a different congregation, no vehicle, etc). Often the issue of forsaking the assembly is easily solved when a simple phone call or a meeting in person to talk about it clears up any misunderstandings. When calling it is important to be kind, humble and clear about why you are calling.
Every Christian is called to help other Christians, whether they are a part of your local congregation or not. If you know a brother or sister is struggling, it is your responsibility to help them. "Brethren, even if anyone is caught in any trespass, you who are spiritual, restore such a one in a spirit of gentleness; each one looking to yourself, so that you too will not be tempted. Bear one another's burdens, and thereby fulfill the law of Christ" (Galatians 6:1-2; see also James 5:19-20; I John 5:16-17).
If the brother shows up at an assembly, or if you see him in public, then take the opportunity to talk to the brother. Don't accuse, but ask him the reason why you have not seen him lately, and how you can help him meet the command to assemble. Then work from there on helping him solve the problem.
Withdrawing from a person is done when there is no remedy left and the person is a lost Christian, given over to their sin and won't repent (I Corinthians 5; I Timothy 1:20).
I hope these steps encourage you to investigate further and find an opportunity to clear up the issue of why they are not attending.