Five-year-old Callum is autistic and non-verbal.
At nursery, he received weekly one-to-one speech and language therapy to try to develop his speech.
His family says he was making good progress and when he started primary school last summer they were led to believe the support would continue.
Instead, it stopped, and they are now scared he may never speak.
Callum is one of thousands of children struggling to access the right support with speech.
Glenn Carter, the head of the RCSLT in Scotland, said the report highlighted the importance of embedding SLT in nurseries and supporting children before they get to school as well as once they are there.
He said the provision of SLT was "variable", with excellent examples in some areas but low levels of funding in others.
Mr Carter said this led to those most in need not getting the resources they require.
"There are kids experiencing significant challenges in school and that causes behavioural issues, which in turn causes stress for teachers," he said.
"We feel that by positioning speech and language therapists within education we can solve quite a lot of the problems that Scotland is facing right now."
Teachers say that since Covid more and more children are starting primary school without being able to communicate verbally.
Recent figures from Public Health Scotland show that last year more than one in 12 children recorded a delay in speech and language by age five.
Pre-pandemic (2018/19) the figure was only one in 20.
The Scottish government said it had invested in a programme to support early intervention in speech and language, helping to build confidence and capacity of staff working in early learning and childcare.
A spokesman said: "As laid out in the Programme for Government, we will publish an Early Years Speech and Language action plan by autumn 2025, which will set our practical steps to support children's early speech and language development."
A North Lanarkshire Council spokeswoman said: "There are UK-wide challenges in recruitment of speech and language staff, and we continue to work closely with colleagues at NHS Lanarkshire to assist in mitigating these."
Pauline Downie, from NHS Lanarkshire, apologised for any distress its letter had caused.
She said: "The intention of the letter is to give parents an opportunity to contact us to discuss any concerns they have regarding the speech, language and communication support for their child."