Problem-solving methods in maths have changed over the years. Because my boys were taught maths in a different way to me, it sometimes made it difficult to help them when they struggled.
If your child is already finding maths hard, he might find it even more confusing using methods that he does not use in school. In time he will most probably learn the techniques you know, but until then I would stick to the ones he is familiar with.
I suggest you start by finding out exactly what methods the children are using in class. You could ask your child to teach you. Talking the process through with you may well clarify it for him. I would also be inclined to ask his teacher to show you the working out methods they are using. This might also be a good time to talk to her about any difficulties your child might be having. She may suggest some other ways for you to support him.
Many schools run numeracy workshops for parents. This is an opportunity for teachers to show the progression of mathematical skills the children will be expected to learn and the sequence of age-related problem-solving methods that they will use. In my experience parents find these workshops extremely useful. If this is something your child’s school does not already do, it might be worth asking if some workshops can be set up. Given that methods do change, I am sure you won’t be the only parent who needs this information in order to support your child.