Majlis (or Mejlis; Arabic: مجلس, pl. مجالس Majālis) is an Arabic term meaning "council", used to describe various types of special gatherings among common interest groups be it administrative, social or religious in countries with linguistic or cultural connections to Islamic countries.
You must be sure that you and your children will always follow Ahlul Bayt (AS). Don't take any risk in this crucial matter. I advise you to invite him to read few books written by Sunni scholars who became followers of Ahlul Bayt (AS) after researching Sunni and Shia books e.g The I was guided by Dr Samawi Tijani and The Miising Link etc.
After reading such books and being able to ask questions to Shia scholars, he should be able to decide his following Ahlul Bayt (AS).
Ideally, and traditionally, religious work should be done first and foremost as a service to Allah. It doesn't not mean that one cannot be paid for it, but just that money shouldn't be the main goal. If someone's main goal is money, there are other professions which are more lucrative.
Religious work - like art or sports or other endeavors which require a lot of dedication - has the highest quality when it is done out of sincerity and love for what one is doing. When it is done primarily for financial gain, it loses something. Also, when it is done with money as the priority, the speaker or reciter loses their freedom to speak the truth as they see it and can begin to feel compelled to please the people rather than please the Truth.
There have been some notable scholars throughout history who took no money for their efforts.
However, on the flip side, it is good for communities to assist those who serve them financially. When a community hears a speech or majlis, they only see the hour or two of the program. However, the person delivering it may have had to take time off of paid work to prepare or travel, invest time and money in training and books and developing speaking skills, etc, etc. For instance, if someone takes 2 weeks off of work to travel to give majalis, they are losing a lot of their basic income.
Some people also rely on lectures and majalis for their income, especially because sometimes people who work full-time in the religious community are not in a good financial situation. Often it is the people who are the most reliant on receiving hediyas that are shy to ask for high amounts or specific amounts. So it is good to be thoughtful of that and to be generous within the range of what is possible.
As far as the permissibility, it is permissible just like any lecturer who is allowed to demand his fees but it is advisable for those who serve religion to avoid high charging. Most of our scholars teach people with out any charges. Yet it is not Haraam to charge but reasonably.
Most importantly, we need to appreciate and thank Allah (SWT) who granted us His guidance and did not leave us away from the Truth of Real Islam of Muhammad and His Ahlul Bayt (AS).
There are few suggestions :
1. Reciting of Ziyarat Aashura is very great every day or when ever you can.
2. Talking to and inviting people to the Truth of Imam Husain (AS) in any suitable way which should be wise and helpful in making people realize the fact of the Real Islam of Imam Husain and distinguish it from the corrupt religion of Yazeed and his gangsters.
3. Gifting good deeds to Imam Husain (AS) and his family and followers.
4. Serving the mourners on Imam Husain (AS) and the religious activities.
5. Reading more about he life and sayings of Imam Husain (AS and passing the information to people.
It is allowed to use or rent or hire for religious activities including Namaz, Majaalis, lectures etc, a place which was or is used in un-Islamic way on other times. It is like the road where we walk on, which is used by good people and bad people. Every one is responsible of his acts.
No doubt, we need to be sure about Taharat in the place of prostration (Sojood) of Namaz.
Musical instruments and alcohol related items should be removed during the religious gathering to keep the respect of the religious activity.