I agree with the other moms who have posted so far...
Have your son and husband spend lots of one on one time together. Maybe they can schedule a time every saturday or sunday tha they can go to something together - just the boys. it can be something simple like going for a walk (through a forest preserve can make it seem more special) or to the park, raking leaves (its almost that time!), or making breakfast in the morning (on a weekend) for the family. Maybe they could go for a bike ride together or go feed the ducks at a local pond.
You could also try making one part of the everyday routine something that dad "owns", like another mom suggested. It should definitely be a fun part of the day.
Building a strong relationship will take time and may seem like its not working.... just keep trying at it and eventually you will notice the changes. Everything will work out if you give it a little more time :)