My 21 month old son has lots of words. He had ~50 at 12 months and that's when I stopped counting. He has nouns, verbs, adjectives, pronouns, uses plural and tenses correctly but he doesn't form sentences. He learnt the alphabet at 15 months and knows his colours. But when he's talking he just says one word at a time and waits for my answer or further question. Sometimes when he's VERY frustrated he'll say multiple words like 'more peaches now!!!' But it doesn't happen often. When I ask him to repeat me saying multiple words he'll just give me a synonym. Is this normal? I'm starting to worry because it seems unusual