My teenage DC with ASD took a flight to France last December. Managed without assistance one way and was unexpectedly offered assistance on the way back. I don't know anything about Rayanair, except they would need to comply with legislation. I researched the issue and it seems there are two types of assistance available.
First, you can have most needs met by your airline if you choose their option of paid assistance for children and teenagers travelling alone. I researched BA, Lufthansa and Air France and the offer is similar. It works for children up to 18 years of age 40-50£ one way. The airline person would accompany him 1:1 through departures, assist with priority boarding, keep an eye on his needs during flight, offer games/entertainment in flight, bring him to the gate at the arrival. The service and flexibility would depend on the airline, but they would not modify any of the airport safety procedures, like taking buggies out. They say they would try as much as they can meet the needs. The key is to contact the airline and explain the needs in the context of purchasing assistance for a minor traveling alone. .
However, the legislation in EU requires the airports (not airlines!) to provide free of charge assistance to disabled passengers. The key words are "Special assistance". The airlines are paying for it and you can only pre-book through the airline by requesting special assistance, or present yourself at the special assistance desk at the airport well in advance of boarding. This special assistance is provided on behalf of the airport by a company that would ensure the same assistance in the airport of arrival i.e. taking from the plane to arrival hall. Try to find the info about special assistance and procedures at the airport website. Heathrow specifies their practice in detail.
The cautionary tale here - typically this is interpreted as needing a wheel chair. When requesting this through the airline, BA specifies that they offer free of charge special assistance to passengers with learning difficulties (i.e. not needing a wheel chair). However other airlines don't even have an option of requesting any assistance except for a wheel chair. We had a difficult discussion with one airline where they told us that they can't book special assistance if the passenger can walk/ doesn't need the chair; that the airport person is attached to the chair and if my DC would not sit in the char the person would walk away. On the argument about broader interpretation of disability they said that if a person has mental disabilities and unpredictable behaviours or reactions, they have the right to refuse boarding because that passenger would be a safety risk in flight or in case of evacuation. They have full discretion to deny boarding. I know this is infuriating, but we didn't want to start another tribunal. You could say we caved in to this intimidation and DC with ASD walked to the gate alone. It was a success. A milestone in independence.
However, on the way back, with the same airline the airport people accepted out explanation and accompanied DC to the gate. So basically it depends on your luck - the people you come across. I know it feels very uncomfortable to take any chances with AS DC.
If you feel your DS really needs help, talk to the airline, but be careful how you articulate his needs and disability.
Look at Stansted and Rayanair websites for info about special assistance.