Experience from my bilingual family set up (5 year-old child):
First of all, I don't think you should worry.
Second, I don't think you need to look for any pediatrician advice at this point.
Third: the situation may me more related to the parents than to the child. If both parents work full time, and pre-school is in English, the child will have to work a little harder to make sense in both languages, but he or she will survive very well....
Fourth: anxiety to speed things up may actually slow things down... So take a deep breath, be patient and don't be negative to your son.
I don't think correcting the toddler is a good idea in general, just translate in the right way after improper use of the language. Imagine this converstation:
"Mom, je parle English!
No, that's English and French!
Either speak English or Cantonese or (...) with me."
This kind of correction may inhibit his language development. He does not need a teacher to correct, but a parent to help.
He will do ok (based on your short description, maybe life is more complicated).
There is nothing wrong with "I want alimento". "That's right, it's lunch time, I want some food, too."
If you can, spend some time (a few weeks) back to your native land where he can find other children speaking the same language. They need strong community (at his age, the community of the little ones) support to speak both languages in the long term. As adults in multi ethnic families, the people who get upset about their bilingual childhood are the ones who actually didn't have the bilingual experience (some parents prefer to speak in English than native language...). They wish they had had it. The ones who had it, it's either a normal thing of life or a plus (as adult, not teenager....).
We had spent a few weeks in my native country 2 and half years ago. That was fantastic and made miracles in her ability AND desire to speak in my native language. I just came back from a 10 day trip, and her language also improved. My daughter was fine tuning her English (specially her "a" sound as 'apple', 'cat', and this dragged the other language... The trip helped again. I don't know if she would fix the second language anyway (I just have one life :), but her second native language is back to her normal.