How exciting to have a child begin middle school! I am sure that both you and your son are feeling a mix of excitement and trepidation, so I want to make sure that I address both of the questions that you pose here.
First, "Is inclusionary learning effective for students with learning disabilities?" Absolutely yes! All students can thrive in an inclusive classroom setting. In addition to being exposed to developmentally appropriate academics, students in inclusive classrooms learn vital character skills such as kindness, empathy, patience, cooperation and so much more. Students with and without disabilities benefit from recognizing that there are a wide range of abilities in all subjects.
But you also asked, "will being in mainstream classes make things harder for him". The answer is maybe, as it really depends upon teachers at this new school. Successful inclusive teachers are those who understand a broad range of abilities and have the skills to differentiate instruction to meet each student's needs. It is also these teachers who will help the children to develop meaningful friendships with one another. It is wonderful that you are asking this question now, as there is plenty of time to advocate for your son before school begins. Don't be shy in sharing the strategies that help him find success and speak often about his strengths in addition to any weaknesses. Help the staff at his middle school to recognize that all students have gifts and than an inclusive class is an opportunity, not a challenge. Only you can know, but I hope that the school has recommended this placement because they know that the teachers are motivated and prepared to be inclusive.
Best of luck!