1. You stated that your son does Mathematics well, it means that he is good at numbers, has the concentration for working out sums, as the subject does not have texts to be read and learnt.
2. Whereas other subjects there is a lot of content wherein the child needs to read and learn. Your son is facing problem in reading, understanding and reproducing. What you need to do is first identify the problem where your child finds it difficult. whether he is able to read the sentences, or not.
3. Is he able to read fluently, is he able to read and recollect. You need to check it. If the child has a problem in reading the sentences he/she will be unable to memorize. Fix a Schedule for learning.
4. If the child cannot read then he/she would be distracted may not like to study, and keeps disturbing. The child should be taught with patience. Take the child in confidence, appreciate this ability in Mathematics.
5. Make him understand the importance of other subjects for his progress. Reward him with small gestures
6. Know the child, allow the child to read and memorize in small parts. Do not force him to learn all the content at once and repeat them without seeing. The child may be having a problem with reading, unable to grasp with words and sounds.
7. Talk to the child why he is not able to learn the other subjects since he is in class-5 he would be able to tell. Do not compare his academics with his sister. Try to involve both of them together in learning, let your daughter read and make him repeat.
8. Tell them that you would reward them if they both studied together. Do not beat the child as it may create fear in the child. Be caring and use simple flowcharts or diagrams to make the child understand the concepts.
9. Do not make the child to learn a large portion at a stretch. Break it up into smaller bits and make him learn. Give more of practice and repeatedly. The child may not learn fast, do not expect results fast. It takes time for the child to pick up.
10. If the child is having a problem studying at home then arrange for a tutor for learning. Talk to the child's teacher, find out the problems faced by the child, in what areas, he is not working upon. Get guidance from them, try to implement. If it's writing then make the child practice writing and learning. If it's reading makes the child read every day to become better. Make it a habit.
11. Create an atmosphere for their comfort for study time. Avoid distractions.
Finally allow the child to get adequate rest, sleep and play time too. Provide him with good healthy food to stay fit and mentally alert. If the child has a problem then we need to understand and provide the best possible help for the child to do well in their studies.