Bilingual child

Hello, We are parents to a nearly 20 months old boy. Both of us-mom AND dad- are Israelis AND speak hebrew but we live in London AND our son goes to a British nursery AND the only language spoken there is English. At home we stick to Hebrew. Our son cant say even one word, not even Ema OR Aba. We are very concerned about it since he would not even repeat common parts of words such as Ba Ba Ba OR Ma Ma Ma. We eliminated any hearing problem since he will respond to someone calling his name OR to a well known song OR tune he likes. We are a bit worried since we are really trying to repeat certain words AND sounds but some times he just does not seem to be interested in learning AND he just becmes impatient AND upset.. Generally speaking he understand quite a limited things we say to him like "where is the ball?" etc Physicaly, he is very well developed - walking, running, playing. What should we do? Maybe switch to English at home just until he will gain confidence at least in one language AND then go back to Hebrew? give up one language? Or should we go to someone proffessional to check certain things? By the way, WE CAN READ AN ANSWER IN HEBREW. Sorry for the length of the Message, Thanks