My child as cystic fibrosis, how likely will he get a lung infection caused by candida?

Somewhat: The main risk for candida in patients with cystic fibrosis are for those patients who take inhaled drugs (such as combined therapies - advair, symbicort (budesonide and formoterol) & Dulera or monotherapies such as asthmanex, qvar, flovent). The inhaled steroids decrease bacterial counts in the mouth and increase the risk of fungal/candidal overgrowth resulting in thrush.

Possible: Depending on your child's lung function and airway clearance compliance, the chances vary somewhat. Candidiasis is rare in pre-adult sputum culture data sent on the CF foundation, except in children with very poor lung function tests who have already had multiple other infections.

Sometimes/immunity: Candida can be associated with colonization of the airway and mouth in cf. The fungus may live in the airway without actually causing an infection. If there are inhaled drugs or medications which affect local defenses, then the candida may result in an infection. A reference for your interest is: http://www.Ncbi.Nlm.Nih.Gov/pubmed/21067323. With a lung transplantation, patients are more prone.