Firstly, you boar is far to young if he is only that weight, he needs to be at least 10 months of age, you do not want the boar to "overstretch" himself at such a young age as it will put him off for the future. Dont let him become overfat or to lean to work, its a fine line getting him just right.
Don't run him with your gilts, let him be in a pen near your gilts so he can either see them/hear them/smell them, he will bring your gilts on and also will make him more keen to work. Take the gilts to him in his pen and let him serve them in his domain.
Sometimes very rarely the sheath is to tight and the penis cannot extend out, if this is the case, its a job for your vet to cut the sheath or have him culled.