Hi Beata
Sounds like he is on target to me.
When I worked special needs it was if they were not putting sentences together by 3 yo. I had a friend whose child turned 3 and was just saying "Mama" type words. Everyone had told her he would catch up, well I said ask the MD. He of course was upset.
Some other guidelines are
18 MONTHS: should be able to understand a variety of words and should be using a few single words. The child should be babbling, understanding simple questions/statements such as "where is your nose?, and "give me".
Your son is not quite 2 yet but
2 YEARS:words should be combined into two and three-word phrases and sentences, such as "more milk", "all gone", "my turn". The child also understands "where is mommy/daddy?" and simple directions such as, "get your coat". Two year olds understand more words than they can speak. A two-year-old understands approximately 300 words.
3 YEARS:can follow simple directions such as, “time to take a bath,” "tell him your name.” he can also put an object in, under, or on top of a table when asked. She can also answer simple questions about objects such as “which one is bigger?” By age three a child understands approximately 900 words and speaks 200 words clearly.
4 YEARS: can follow two-step directions such as "close the book and give it to me". She also knows her first and last name, can answer reasoning questions such as, "What do we do when we're cold?", and can tell a short story such as, "two kids played ball." Sentences are usually 4 to 5 words long. By four a child is giving directions such as "put my shoes on" and asking many questions. A four-year-old understands 1500-2000 words and can use the following pronouns: he, she, you, me, I, mine.
The more important information is the progression that takes place, and that understanding is not speaking.
God bless you and give you peace
K. SAHM married 39 years --- adult children -- 38,33,and twins 19