Yes, cutting back right after it blooms in the spring will make it fill out. The general rule of thumb is to remove up to 1/3 of the plant, but if the leaves are at the end, I would just cut of the ends with about 1/3 of the leaves.

Also, if it has too much shade, cut out some of the lower branches to give it more light. Make sure the soil is well drained. That is very important. Also, you may want to use a good rhododendron fertilizer like HollyTone after it blooms in the spring.

Make sure it is mulched. Do not cultivate around the very shallow roots.

Here is a link that might be useful: How to grow rhododendrons and azaleas.