Hi SabineMum,
I'm a speech and language therapist so can give you some advice. I particularly noticed your post as we presented training to a nursery about bilingual language development this weekend!
It is normal for children learning two languages from birth to have a delay in their talking, usually around 6 months or so behind their peers. There can be a 'silent'period where they are not yet talking but do understand everything. Their talking then comes on well and there are no long term issues.
Do keep your child exposed to both languages and try to keep the exposure consistent e.g mum always speaking French, English at nursery/with dad.
It is good that your son is putting some words together e.g. I want + word, this is what we would expect from a two year old and shows his language is coming along even though his vocabulary is limited.
At an assessment we would look at whether a child has delayed development in both languages. If so we would give advice on strategies for parents to encourage early language development. Most important is to give a good language model of words the child 'would use if they could' and give him lots of time to respond or show you what he means if he can't say it-to reduce frustration.
Feel free to contact me for any further advice, this is a common question!
Marianne
http://Www.childrenstherapies.co.uk
02086737930