The Design and Technology Teachers Association (DATA) published the following advice re:MDF
“Is MDF safe to use?
Much media attention has been drawn towards MDF and the hazards associated with its use in schools. The D&T Association has no evidence from either HSE or the Education Service Advisory Committee (ESAC), which leads us at present to call for MDF to be removed from use in schools. We however welcome the decision by the HSE to fund a research project at De Montfort University. Hardwood dust is classified in COSHH as carcinogenic, and softwood is now a suspect carcinogen, although not defined as such in COSHH. Thus under risk assessment procedures substitution by natural wood may not eliminate the problem. The D&T Association’s advice to schools: 1. A high level of cleaning of the teaching environment must take place to reduce to a minimum the dust in the environment. 2. All machining of MDF, and other timbers, should have good quality and effective dust extraction. 3. Rooms should be well ventilated when working MDF by hand tools. Rotary sanding discs or machine sanders with no dust extraction must not be used in schools. Beware of companies trying to sell these to schools.’
So the bottom line is that provided adequate ventilation is used and careful cleaning up undertaken the risk is very low. Certainly the OP’s son is likely to have had a low exposure