Any suggestions on how I can increase the weight gain in boer goats

You may need a higher fat content than protein content. You can add veggie oil to their feed but alfalfa pellets or an alfalfa mix hay would not hurt either.

Also remember that muscle development is also important so make sure your goats have climbing contraptions as climbing helps build up body mass.

Check at homesteadingtoday.com They have a forum devoted to goats, and the people there are very helpful.

A foolproof activity for me in gaining weight has always been drinking a six pack after every day of work. I gained 18 pounds in a month!