I'm sorry this is worrying you so much, but he actually sounds pretty normal to me. I don't think it's that unusual to use words once then not again for a long time. It may just be a 'choice', rather than an inability.
At 18 months my son had maybe 5 actual words and few sounds that he used. By 2 he was talking in 5-6 word sentences.
do you have concerns about his comprehension or hearing at all? Any other reason why you're worrying about this? There certainly is no harm in getting him checked out if you want to. I hope you can get some reassurance.