Hi G.,
I am also French and speak it exclusively to our sons (25 months and 6 months). I don't think it would hurt to get your son evaluated, especially if it makes you feel better, but from I've been through with my son, what you are describing sounds normal.
Our 2 year old was the same way at that age and all of a sudden in the last 2 months, his language drastically developed. Of course, I'm not sure it is AS developed as another same age child in a unilingual household, but definitely a lot better than it was a few months ago. All of a sudden, around 22 months, he started repeating what we said, and also saying words we didn't even know he knew! His vocabulary started to expand drastically in a period of a month, and he went from saying a few basic words and sounds, to making 2-3 word "sentences", sometimes a mixture of french and english (my husband speaks only English)...for example, he'll say "encore milk, maman".
His receptive language skills were always right on track for his age (like your son, he would understand whatever we'd ask of him in either language, like "go get your boots" " va ouvrir la porte au chien"...etc. without hesitation), which made me feel a lot better.
We've actually moved to NH from VA about 6 months ago, and neither our pedi in VA nor the one here (and they don't know each other ;-) told us we had anything to worry about or recommended we have him evaluated.
Feel free to email me if you have more questions, I've been there, I know how easy it is to worry about language, especially when you hear other children the same age speak a lot more, but try not to, at least for now. It should all get better soon!!!
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C.