Robin Thicke's ex-wife has accused him of 'punching' their six-year-old son.

Robin Thicke and ex-wife Paula Patton are embroiled in a nasty custody battle, with Patton accusing her former husband of physically abusing their six-year-old son.

Celebrity news website TMZ report new legal documents filed in the dispute allege son Julian told school teachers his father hit him a number of times and even “punched” him.

In response, the school reported the incident to the L.A. County Dept. of Children and Family Services (DCFS) on January 3rd, and an investigation has since begun.

In light of the allegations, it is reported Patton, 41, began denying Thicke access to their son, filing an emergency order seeking to restrict Thicke’s contact with the six-year-old, claiming that “Julian is scared of you”.

The singer currently sees his son overnight every Thursday, Friday and Saturday, as part of the duo’s current arrangements, news.com.au reports.

Me and my boy!

A photo posted by Robin Thicke (@robinthicke) on Oct 31, 2016 at 12:39pm PDT

“Over the past year, I have become concerned about [Thicke’s] drinking and drug use, as well as the forms of punishment he is using to discipline [Julian],” Patton stated in paperwork submitted to court.

However, a hearing on Thursday saw a judge deny Patton’s request to change the custody arrangement, ruling that there was insufficient evidence that Thicke’s behaviour was causing “great or irreparable harm” to his son.

Thicke testified that while he occasionally “spanks” his son, he is not abusive.

“I have always used progressive discipline with Julian; primarily the use of ‘time outs,'” he testified. “One a very rare occasion, and only as a last resort, I will use light spanking, but it is consistent with the law – open hand on the butt. This is the type of discipline to which Paula and I agreed during our marriage.”

The DCFS case is still ongoing, and just another dispute in the couple's bitter split.

According to Thicke, the accusations came about because he decided to block Patton from attending his dad Alan's funeral last month.

"Paula did not have a positive relationship with my father and often made negative comments to me about my father. As such, she was not welcome at his funeral," he says in court documents.

Thicke began dating Patton at the age of 16. The couple were subsequently married in 2005 and had their son, Julian, in 2010.

They separated in February 2014, after 21 years together and almost nine years of marriage.