hello,

It's okay to be weak. You can inspire him to read books for more times and solve simple questions then move to difficult questions. With practice your brother will do good.

you can also monitor him and push him a bit if he relaxes himself. Brain is always lazy and doesn't work until it gets hit by some strong cause. You can provide him one.

for army and other government jobs, a percentage above 60% is mostly eligible. if he score above 65 or nearly 70 then that is also good. he will be eligible for many options. but for that he has to do graduation like Btech,BA,Bsc. Mostly Btech is easy one as there are many scope in it. Engineering provides more scope than any other stream and also government jobs.

above all, your brother has to practice physics,chemistry and must have clear basics and concepts. This comes when alot of practice is done. getting bored in one read won't help. on the part of numerical, it is not easy to solve in 1st attempt. it takes lots of effort to understand a questions and solve it. Every thing is like a baby step. small and continues step will obviously help your brother to improve in weak subjects

Hope it helps!