Hi V.,
I admire your commitment to teaching both languages to your children. And first, a few personal observations:
* It seems the girls in my son's daycare center are ahead of most of the boys of the same age in their language development
* My husband didn't talk--at all--until he was 5 years old. Professionals told his parents he was retarded adn should be institutionalized. His mother vehemently disagreed and fought the diagnoses. He WAS placed in a developmental enrichment program, where he met his friend JJ, who said only swear words. My husband said his first words, "mother *ucker" while on vacation, much to another mother's horror. My mother-in-law was overjoyed that her son was speaking, and taught him to say "mother trucker" instead. PS--20+ years later, my husband completed an Ivy League bachelor's degree, then a master's degree, then a law degree. He is one of the most eloquent speakers and writers I have ever met.
Now--in case you want to pursue further evaluation or he continues to have difficulty as he gets a bit older:
Knowing nothing specific about speech development, but as a family life and mental health professional who frequently makes referrals for occupational therapy, neuropsychological, bonding, and other types of evaluations and routinely interacts with those professionals, I would advise you to seek a second opinion with a speech therapist who you know from the get-go has experience working with bilingual families. If you hear the same advice, it might be worth further pursuing evaluations to determine if your son has a disorder of the part of the brain involved in the cognitive and/or physical aspects of speech production (as opposed to receptive language). I happen to have two friends whose sons both struggled with an inability to physically produce the speech sounds that they could mentally conjure, and speech therapy helped them greatly. It was not a confusion issue--it was an issue of their brain not being able to "tell" their muscles to move the right way to produce the speech sounds that they were trying to make. Both boys experienced a high level of frustration at trying to be understood, had wicked temper tantrums tied to their expressive language difficulties, and became more shy and withdrawn as a defense against engaging in social situations that would be frustrating or embarrassing to them. And in both cases, when their speech therapy started, they blossomed both expressively and emotionally, boosting their confidence and self-esteem. And it had nothing to do with "confusion", but rather, ability to produce the sounds.
Good luck to you! Your son is lucky to have such devoted and conscientious parents.