Rush him with your hands/arms held up high,blocking your face,and try to get him to the ground as quick as possible.A boxer requires his feet to be planted on the ground to generate power.Take away his upright position,and he will no longer be able to rely on boxing skills.Also,while in close,hit him with other things besides punches,like headbutts,elbows,and knees.Boxers get real rattled by that type of shit,as they are used to fighting within a very limited section of rules.Mix up a lot of wrestling with striking,and you'll have a good chance at holding your own.Just don't get careless and leave yourself open.