Your saggitarius needs focus on attention. Let me tell you your
scenario happens to virtually almost all relationship (90%). All
relationship will walk out of honeymood period eventually,
regardless of how much attempt is to hold it. Under Love Life
Cycle, good relationship generally stay within the growth stage.

Its NORMAL for people to 'slack' gradually as their
relationship advance - because motivation isn't as strong as
previously when he has to chase you.
Try to find life outside your boyfriend. One of the top few rules
to maintain a good overall relationship is to have life outside
your partner, for who wants a mate to be dull and so dependant on
him/her? With decent social circle outside him, he has to
constantly prove and motivate himself to work towards this
relationship, which is doing good for you love. 
In theory explanation, the Law of Diminishing Effort state that
partner will do minimal to upkeep a relationship, while maintaining
security. If upkeeping a relationship and still feeling secured in
your relationship means sms thrice a day against once in three
days, if both still bring about similiar effect - he will do the
latter. 
Being in a BGR doesn't signify anything. More effort must be work
through to see it blossom. Have a good talk with him and let him
realise this point of you. Communicating your thoughts effectively
is one of the first step you have to do, at least allow him time to
get to know the real you, instead of exploding one fine day without
him knowing what's wrong. 
P.S: sillyme's advice is exactly what I wanted to say as well.
Check it out - its a good way to spend your time.
Cheers