Should I try to tell him how I feel through a gift?

i think, if you stayed friends and have a good relationship and his girlfriend feels secure,then your card could sound similar to what you've wrote here. And the t-shirt idea i think is fantastic! if you haven't stayed friends and you don't communicate much and his new gf knows you just as his ex, not a friend, then stick to simple congrads as all other stuff can make lots of hle between you, him and his gf.