Hi Claire,
I am a trained bilingual speech and language therapist (English & Spanish)so I can give you some information on bilingual language acquistion in general, however if you have more specific questions send me an email. A bilingual child may appear 'slower' than a monolingual child as they are acquiring 2 vocabularies, 2 grammars, etc. If he appears not fluent, is that compared to other bilingual or monolingual children?
As they are learning both languages, the child may codeswitch and stick in a french word in the sentence when he can't remember the English word or hasn't learned it in English yet. If this happens you can just repeat/model the correct way but careful not to correct the child, just stick to 1 parent speaking French only and 1 speaking English and maintain a positive language enriched environment. Eventually the child will become more fluent, as he attends school in English only, you might see a quiet period, where he is not as talkative as he might be processing all the learning and then eventually he will become more expressive. Some researchers say it takes up to 7 years for a child to develop both languages. He may need more time.
If you feel is he not acquiring both English and French then he should be assessed in both languages to determine if he is delayed, it can not be determined by only assessing him in English as he is not a monolingual child. A disorder is determined if it is seen in both languages otherwise it is an English as a second language difficulty which would mean more exposure to either language is needed and not direct speech or language intervention. I hope this information is helpful, feel free to email again should you have anymore questions, coughlansinead@yahoo.com