I noticed that your son is very young for class IV. He is only eight. Ideally he should have been in class 3.
I have noticed that even a six months difference in the age of kids, contributes to the difficulty in children being able to pick a few things in particular as compared to other topics.
Kids love to pick what they love or what is interestingly taught.
My kids hate when I start with their school books and revisions.
But every 10 min they ask me, when will I do the fun study with them.
My kids go to diffrent schools.
One has started hindi in nur, where as the other one who is on 1st has started now. The younger one is still doing ABC.
He was youngest in his class when he was in Nursery, in Kiderbrook. So I decided to allow him to have less pressure and made him do Nursery again in DPS.
He is not very keen on school type studies.
I read stories from Ready to Read - Nick and Claire Page
Or any book which has maximum 20 lines. I read it first with my finger under each word. That makes the kid remember sound unconsciously. Then I ask the older one to read,in the same fashion. Once he is done, then I ask tiny question which the younger one has to answer first in one word or a sentence, whereas the older one has to give a correct answer which is a full sentence with pause and punctuations. I used to make him repeat the answers earlier, but, now he has learnt it.
Surprisingly, my younger one has started answering in full sentence too now, even though he is not expected to do so. :)
Then, I ask the older one to pick out all Nouns and Verbs as he is doing that now in std 1.
Then the younger one has to either give a rhyming word or meaning or just the starting sound and its first letter.
The older one has to make a sentence of a few words.
I draw pictures of a few words from that story and let them tell me anything they can. Since they are two, they compete at times to give me the correct answer first.
Another thing important I have noted is that the TV. I have made it compulsory for them to watch only in english. They would change it to hindi the moment I got busy. SO now I make them watch one hindi and one english. :)
I know of few classes who teach vocabulary but they they charge a bomb to teach one word per week. !
I think reading is best and making them hear english is the most natural way to pick it.
Do not worry, every child has different times when she/he gets it. I used to get things and concepts only after I passed a class and went to the next one :D.
Infact, I am rediscovering science and concepts better with more practical application around me when I am trying to teach things like magnetism, electricity, pressure, vaccum etc to my kids.:)