You don't say whether you are teaching this child individually or as part of a class. My first question would be: is he doing as well as the others in his class? or is he performing better or worse? What do you think of his achievements compared to others of his age? Is he reaching the grades you would expect for someone of his age?
Your question shows that his parents put a lot of emphasis on tests and grades you also mention that he forgets what he was taught in tests. It seems to me - and maybe I'm wrong - that this child is under a lot of pressure to achieve. I think it's important that a child at the age of 8 has a positive experience when learning a language. An early positive experience will motivate him for the rest of his life. It's also important to remember that learning a language is not like learning other subjects. There are a lot of things to consider: vocabulary, pronunciation, willingness to try out the language, etc. A child of 8 is still learning his/her own language and so may not be able to cope with spelling and writing in another language. Also, some children are shy and naturally reticent and this can affect their spoken English. Often children understand a lot more than they can produce.
I understand that parents can put pressure on children to achieve and that this can reflect on the teacher. They've obviously spoken to you about the child's grades. I think you need to talk to them about the factors involved in learning a language and how their child is successful even if he isn't getting the grades they want. I know this can be difficult, but do give it a try. To save the child from unnecessary stress, you could make the tests simpler and base them on understanding not production of language. The aim being for the parents to see progress. Not everybody would agree with me on this but I really think the issue here is the parents' insistence on grades and not the child's performance and certainly not your teaching abilities.
It seems to me that you are doing the right things: you are checking his understanding and reviewing work in class. You are correct in saying that review is necessary, especially at this young age. Going back over the same language but in different ways is essential. You can do songs, jazz chants, games and stories that all use the same language.
There isn't much you can do if somebody has a bad memory; all you can do is to review the work; praise and encourage where appropriate. It's important to remember that a child's short-term memory is well developed but at the age of 8 the long-term memory will be less developed, long-term can mean days. I think if the pressure is taken off him, he will show more progress. I think a grade of 80 is a lot to ask for in a child of this age.
As for forgetting his homework, I think the parents need to be involved in this. They need to ensure that he does his homework and remembers to bring it with him. Can you speak to them about this? and does the boy have a notebook where he writes down what needs to be done?
I hope this will be of use. If you want some more ideas about teaching primary, please write in again. But in this instance, I really feel it's the parents' perception that is off the mark and not your teaching skills or the boy's abilities.