Academically LKP is good enough for your son to get a place at a local university if your son is willing to work hard.
It worths for your son to try subjects that he likes and is capable to handle. I know a girl who was assigned to the science streams in the old days and she hated science and finally she repeated F.4 for switching to the arts stream and got 7As at HKCEE. Another example, a friend of mine who studied Arts stream at A-level that she could not handle well and finally she had second attempt at A-level but switched to the Commerce streams that she liked so much. Finally, she had great improvement in A-level exam results and was accepted to HKU at her second attempt at A-level and studied Business.
It doesn't matter to change to a lower standard secondary school. The most important thing is to study the subjects that your son is interested in so he can perform well in the longer future. Even he cannot switch this year, he can change school and repeat F.4 in the coming September so it is easier for him to get good results in HKDSE for his future studies.