He said many other former Brethren members now wanted to sue the religious group.
"We're building worldwide momentum for a class action," Forrest told NZME News Service today.
He said the group comprised "several hundred" ex-Brethren members planning to challenge the impact of church dogma and doctrine on people's lives.
"Forced indoctrination, denial of basic human rights, denial of rights to tertiary education - there's a whole raft of things but that should give you an idea," he said.
"All I can say is, there's people involved right across the world and it's something that's being slowly developed."
Forrest said a discussion paper in circulation was intended to foster debate over how aggrieved ex-Brethren could sue the church over alleged human rights abuses.
He said the case would possibly raise claims Brethren used threats of excommunication and damnation to hell to allegedly dissuade members from acquiring tertiary education.
Mr Forrest said any class action would aim to improve the lot of children who were "through no choice of their own" stuck in unpleasant situations.
He said the class action would aim to force "regulation and oversight" of various Brethren activities including education and "discipline practices."
Nobody from the Brethren could be immediately be reached for a response to claims about the possible class action.
But the church, formally known as the Plymouth Brethren Christian Church, has previously rejected claims members were discouraged from tertiary education.
"The motto of 'Learning to Learn' implies the preparation of young people for lifelong learning, and indeed students are actively encouraged to continue formal study after they complete secondary schooling," the Church states on its website.
"A comfortable majority of school leavers are thus taking up tertiary studies...with many 'earning while they are learning' in their endeavour to becoming proficient in their chosen field," the Church added.
The Church also previously rejected claims its schools were not up to scratch.
"We rely on our professional non-Brethren teachers to select whatever material they need to deliver the curriculum within the ethos of our schools," the church said earlier this year in response to a British media story.
A 2011 Employment Court decision relating to a Northland Brethren school said the school did offer secular content, such as Shakespeare's King Lear, in its curriculum.
Earlier this year, Mr Forrest raised more than $663,000 from nearly 18,000 donors to bring Leo home.
But he since faced allegations he had not spent his son's trust money appropriately - for example, by forking out for business-class tickets from Armenia, and asking for $90,000 in his first year as Leo's caregiver.
Mr Forrest addressed concerns about how legal fees in the possible Brethren lawsuit would be paid for.
"At the moment, the process we're working through is all pro bono."
He said suggestions of frivolous spending were mean-spirited.
"My wife and I feel we're being subject to a completely unjustified personal vendetta. And it's having a huge impact on us, you know.
"Once again it's all above board. It's being managed by professionals. We're just doing what we have to do for our little guy."
Mr Forrest said access to regular care for Down Syndrome sufferers was not easy, even in Auckland.
"Medical costs and therapies are running into the thousands," he said.
"We haven't been able to access what we need through the public [system]."
He said other people who had children with Down Syndrome had been in touch.
"They've certainly followed up what the New Zealand system has to offer. While the services are available, the regularity of them is very low. So you might have a particular service, but it only occurs once every two months. When you go private, you can have it every week."
He said there was no need to defend getting the best treatment he could for his son.
"We're getting what we can but we're very strong in terms of an early-intervention approach to Leo, believing that whatever we put in [now] will significantly affect the outcomes when he's college-age. We're making a lot more progress by going private."
Exclusive Brethren
•Formed in the early nineteenth century in England due to dissatisfaction within the Anglican Church.
•Live, work and pray as "Brethren" - a community of families held together by common belief.
•Have no formal hierarchical structure or paid clergy.
•Are distinct from other Brethren groups, including Open Brethren.
