M.:
How wonderful that you and your husband have 3 languages to share with your little boy! My Mexican husband and I speak to our now 5 year old son and 2 year old daughter exclusively in Spanish and they use English with my mom (who lives with us) and at daycare, where they receive Spanish as well. Like you, when my son was younger I would interpret the books in Spanish so he would hear more Spanish than I had books and once he was about 3 (and was conscious of the difference in languages from a visual standpoint) I would read the English books in English and made a point of searching out more books in Spanish whether via the Internet or the local library. Now at 5 he is quite the chatterbox in both languages. Our daughter joined our family from Guatemala at 2 years of age. She understood a lot more Spanish than she spoke but in a matter of 6 months has increased her vocabulary tremendously and her English is coming along as well. (We do notice that a lot of longer Spanish words she cuts off the first syllable - as you well know the words can be a lot longer :)
At any rate, what I am getting at is to just keep up the reading, music, etc. in all 3 languages as you are doing and you will probably find him using actual words closer to the 2 year mark and if not, I think you will still have plenty of time to have him evaluated and receive services if there is a need. As you have noticed, a lot of professionals do not understand bilingualism or multilingualism and therefore assume any delay is due to being "overwhelmed" by all the languages. When we first got home from Guatemala with out daughter it was assumed we would put her in speech therapy right away but I figured I would give her a chance to adjust first and assimilate the two languages and sure enough, while she may be a little behind I think she is well within the range of "normal".
We only spoke English at home growing up and apparently much like your son, I created my own little language so around 2 my mother had me evaluated by a speech therapist who said I would speak when I was ready since I did seem to understand what was said to me. And sometime during nursery school my vocabulary took off and now even my husband can't quiet the chatter :)
I am very passionate about this subject and would love to talk with you more so please feel free to send me a message off-line.
Take care,
L.