First of all, he’s been performing the same routine over the last 4 months, which means his body may have adapted to the specific sets, repetition ranges, and types of exercises he performs, leading to less strength gains. I would recommend him to switch up his routine! He can consider doing more sets per exercise e.g. 3 sets with lower repetitions e.g. 8 repetitions. He can also consider switching weight machines to free weights, which can help target more muscles and larger muscle groups. He can also increase the number of days he trains a week, from two to three. He can also do well to remember to properly warm up, cool down, stretch, and order his exercises properly (from larger muscle groups to smaller).